Provide more widget const constructors. (#9247)

Still many more to go.

packages/flutter/lib/src/painting/fractional_offset.dart

@@ -14,7 +14,9 @@ class FractionalOffset {
   /// Creates a fractional offset.
   ///
   /// The [dx] and [dy] arguments must not be null.
-  const FractionalOffset(this.dx, this.dy);
+  const FractionalOffset(this.dx, this.dy)
+    : assert(dx != null),
+      assert(dy != null);
 
   /// The distance fraction in the horizontal direction.
   ///

packages/flutter/lib/src/widgets/banner.dart

@@ -167,19 +167,18 @@ class Banner extends StatelessWidget {
   /// Creates a banner.
   ///
   /// The [message] and [location] arguments must not be null.
-  Banner({
+  const Banner({
     Key key,
     this.child,
     @required this.message,
     @required this.location,
     this.color: _kColor,
     this.textStyle: _kTextStyle,
-  }) : super(key: key) {
+  }) : assert(message != null),
-    assert(message != null);
+       assert(location != null),
-    assert(location != null);
+       assert(color != null),
-    assert(color != null);
+       assert(textStyle != null),
-    assert(textStyle != null);
+       super(key: key);
-  }
 
   /// The widget to show behind the banner.
   final Widget child;
@@ -215,7 +214,7 @@ class Banner extends StatelessWidget {
 /// Does nothing in release mode.
 class CheckedModeBanner extends StatelessWidget {
   /// Creates a checked mode banner.
-  CheckedModeBanner({
+  const CheckedModeBanner({
     Key key,
     @required this.child
   }) : super(key: key);

packages/flutter/lib/src/widgets/basic.dart

@@ -65,13 +65,12 @@ class Opacity extends SingleChildRenderObjectWidget {
   ///
   /// The [opacity] argument must not be null and must be between 0.0 and 1.0
   /// (inclusive).
-  Opacity({
+  const Opacity({
     Key key,
     @required this.opacity,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(opacity != null && opacity >= 0.0 && opacity <= 1.0),
-    assert(opacity != null && opacity >= 0.0 && opacity <= 1.0);
+       super(key: key, child: child);
-  }
 
   /// The fraction to scale the child's alpha value.
   ///
@@ -108,15 +107,14 @@ class ShaderMask extends SingleChildRenderObjectWidget {
   /// Creates a widget that applies a mask generated by a [Shader] to its child.
   ///
   /// The [shaderCallback] and [blendMode] arguments must not be null.
-  ShaderMask({
+  const ShaderMask({
     Key key,
     @required this.shaderCallback,
     this.blendMode: BlendMode.modulate,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(shaderCallback != null),
-    assert(shaderCallback != null);
+       assert(blendMode != null),
-    assert(blendMode != null);
+       super(key: key, child: child);
-  }
 
   /// Called to creates the [Shader] that generates the mask.
   ///
@@ -154,13 +152,12 @@ class BackdropFilter extends SingleChildRenderObjectWidget {
   /// Creates a backdrop filter.
   ///
   /// The [filter] argument must not be null.
-  BackdropFilter({
+  const BackdropFilter({
     Key key,
     @required this.filter,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(filter != null),
-    assert(filter != null);
+       super(key: key, child: child);
-  }
 
   /// The image filter to apply to the existing painted content before painting the child.
   ///
@@ -206,10 +203,9 @@ class BackdropFilter extends SingleChildRenderObjectWidget {
 ///  * [Canvas].
 class CustomPaint extends SingleChildRenderObjectWidget {
   /// Creates a widget that delegates its painting.
-  CustomPaint({ Key key, this.painter, this.foregroundPainter, this.size: Size.zero, Widget child })
+  const CustomPaint({ Key key, this.painter, this.foregroundPainter, this.size: Size.zero, Widget child })
-    : super(key: key, child: child) {
+    : assert(size != null),
-    assert(size != null);
+      super(key: key, child: child);
-  }
 
   /// The painter that paints before the children.
   final CustomPainter painter;
@@ -292,7 +288,7 @@ class ClipRect extends SingleChildRenderObjectWidget {
   ///
   /// If [clipper] is null, the clip will match the layout size and position of
   /// the child.
-  ClipRect({ Key key, this.clipper, Widget child }) : super(key: key, child: child);
+  const ClipRect({ Key key, this.clipper, Widget child }) : super(key: key, child: child);
 
   /// If non-null, determines which clip to use.
   final CustomClipper<Rect> clipper;
@@ -330,14 +326,13 @@ class ClipRRect extends SingleChildRenderObjectWidget {
   /// right-angled corners.
   ///
   /// If [clipper] is non-null, then [borderRadius] is ignored.
-  ClipRRect({
+  const ClipRRect({
     Key key,
     this.borderRadius,
     this.clipper,
     Widget child,
-  }) : super(key: key, child: child) {
+  }) : assert(borderRadius != null || clipper != null),
-    assert(borderRadius != null || clipper != null);
+       super(key: key, child: child);
-  }
 
   /// The border radius of the rounded corners.
   ///
@@ -379,7 +374,7 @@ class ClipOval extends SingleChildRenderObjectWidget {
   ///
   /// If [clipper] is null, the oval will be inscribed into the layout size and
   /// position of the child.
-  ClipOval({ Key key, this.clipper, Widget child }) : super(key: key, child: child);
+  const ClipOval({ Key key, this.clipper, Widget child }) : super(key: key, child: child);
 
   /// If non-null, determines which clip to use.
   ///
@@ -425,7 +420,7 @@ class ClipPath extends SingleChildRenderObjectWidget {
   /// size and location of the child. However, rather than use this default,
   /// consider using a [ClipRect], which can achieve the same effect more
   /// efficiently.
-  ClipPath({ Key key, this.clipper, Widget child }) : super(key: key, child: child);
+  const ClipPath({ Key key, this.clipper, Widget child }) : super(key: key, child: child);
 
   /// If non-null, determines which clip to use.
   ///
@@ -451,20 +446,18 @@ class ClipPath extends SingleChildRenderObjectWidget {
 /// A widget representing a physical layer that clips its children to a shape.
 class PhysicalModel extends SingleChildRenderObjectWidget {
   /// Creates a physical model with a rounded-rectangular clip.
-  PhysicalModel({
+  const PhysicalModel({
     Key key,
     @required this.shape,
     this.borderRadius: BorderRadius.zero,
     @required this.elevation,
     @required this.color,
     Widget child,
-  }) : super(key: key, child: child) {
+  }) : assert(shape != null),
-    if (shape == BoxShape.rectangle)
+       assert(borderRadius != null),
-      assert(borderRadius != null);
+       assert(elevation != null),
-    assert(shape != null);
+       assert(color != null),
-    assert(elevation != null);
+       super(key: key, child: child);
-    assert(color != null);
-  }
 
   /// The type of shape.
   final BoxShape shape;
@@ -473,6 +466,8 @@ class PhysicalModel extends SingleChildRenderObjectWidget {
   ///
   /// Values are clamped so that horizontal and vertical radii sums do not
   /// exceed width/height.
+  ///
+  /// This is ignored if the [shape] is not [BoxShape.rectangle].
   final BorderRadius borderRadius;
 
   /// The z-coordinate at which to place this physical object.
@@ -501,16 +496,15 @@ class Transform extends SingleChildRenderObjectWidget {
   /// Creates a widget that transforms its child.
   ///
   /// The [transform] argument must not be null.
-  Transform({
+  const Transform({
     Key key,
     @required this.transform,
     this.origin,
     this.alignment,
     this.transformHitTests: true,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(transform != null),
-    assert(transform != null);
+       super(key: key, child: child);
-  }
 
   /// The matrix to transform the child by during painting.
   final Matrix4 transform;
@@ -554,15 +548,14 @@ class FittedBox extends SingleChildRenderObjectWidget {
   /// Creates a widget that scales and positions its child within itself according to [fit].
   ///
   /// The [fit] and [alignment] arguments must not be null.
-  FittedBox({
+  const FittedBox({
     Key key,
     this.fit: BoxFit.contain,
     this.alignment: FractionalOffset.center,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(fit != null),
-    assert(fit != null);
+       assert(alignment != null),
-    assert(alignment != null && alignment.dx != null && alignment.dy != null);
+       super(key: key, child: child);
-  }
 
   /// How to inscribe the child into the space allocated during layout.
   final BoxFit fit;
@@ -591,14 +584,13 @@ class FractionalTranslation extends SingleChildRenderObjectWidget {
   /// Creates a widget that translates its child's painting.
   ///
   /// The [translation] argument must not be null.
-  FractionalTranslation({
+  const FractionalTranslation({
     Key key,
     @required this.translation,
     this.transformHitTests: true,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(translation != null),
-    assert(translation != null);
+       super(key: key, child: child);
-  }
 
   /// The offset by which to translate the child, as a multiple of its size.
   final FractionalOffset translation;
@@ -626,13 +618,12 @@ class RotatedBox extends SingleChildRenderObjectWidget {
   /// A widget that rotates its child.
   ///
   /// The [quarterTurns] argument must not be null.
-  RotatedBox({
+  const RotatedBox({
     Key key,
     @required this.quarterTurns,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(quarterTurns != null),
-    assert(quarterTurns != null);
+       super(key: key, child: child);
-  }
 
   /// The number of clockwise quarter turns the child should be rotated.
   final int quarterTurns;
@@ -656,13 +647,12 @@ class Padding extends SingleChildRenderObjectWidget {
   /// Creates a widget that insets its child.
   ///
   /// The [padding] argument must not be null.
-  Padding({
+  const Padding({
     Key key,
     @required this.padding,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(padding != null),
-    assert(padding != null);
+       super(key: key, child: child);
-  }
 
   /// The amount of space by which to inset the child.
   final EdgeInsets padding;
@@ -703,17 +693,16 @@ class Align extends SingleChildRenderObjectWidget {
   /// Creates an alignment widget.
   ///
   /// The alignment defaults to [FractionalOffset.center].
-  Align({
+  const Align({
     Key key,
     this.alignment: FractionalOffset.center,
     this.widthFactor,
     this.heightFactor,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(alignment != null),
-    assert(alignment != null && alignment.dx != null && alignment.dy != null);
+       assert(widthFactor == null || widthFactor >= 0.0),
-    assert(widthFactor == null || widthFactor >= 0.0);
+       assert(heightFactor == null || heightFactor >= 0.0),
-    assert(heightFactor == null || heightFactor >= 0.0);
+       super(key: key, child: child);
-  }
 
   /// How to align the child.
   ///
@@ -774,7 +763,7 @@ class Align extends SingleChildRenderObjectWidget {
 ///    rather than just centering it.
 class Center extends Align {
   /// Creates a widget that centers its child.
-  Center({ Key key, double widthFactor, double heightFactor, Widget child })
+  const Center({ Key key, double widthFactor, double heightFactor, Widget child })
     : super(key: key, widthFactor: widthFactor, heightFactor: heightFactor, child: child);
 }
 
@@ -798,13 +787,12 @@ class CustomSingleChildLayout extends SingleChildRenderObjectWidget {
   /// Creates a custom single child layout.
   ///
   /// The [delegate] argument must not be null.
-  CustomSingleChildLayout({
+  const CustomSingleChildLayout({
     Key key,
     @required this.delegate,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(delegate != null),
-    assert(delegate != null);
+       super(key: key, child: child);
-  }
 
   /// The delegate that controls the layout of the child.
   final SingleChildLayoutDelegate delegate;
@@ -830,10 +818,9 @@ class LayoutId extends ParentDataWidget<CustomMultiChildLayout> {
     Key key,
     @required this.id,
     @required Widget child
-  }) : super(key: key ?? new ValueKey<Object>(id), child: child) {
+  }) : assert(child != null),
-    assert(child != null);
+       assert(id != null),
-    assert(id != null);
+       super(key: key ?? new ValueKey<Object>(id), child: child);
-  }
 
   /// An object representing the identity of this child.
   final Object id;
@@ -890,9 +877,8 @@ class CustomMultiChildLayout extends MultiChildRenderObjectWidget {
     Key key,
     @required this.delegate,
     List<Widget> children: const <Widget>[],
-  }) : super(key: key, children: children) {
+  }) : assert(delegate != null),
-    assert(delegate != null);
+       super(key: key, children: children);
-  }
 
   /// The delegate that controls the layout of the children.
   final MultiChildLayoutDelegate delegate;
@@ -992,8 +978,8 @@ class ConstrainedBox extends SingleChildRenderObjectWidget {
     Key key,
     @required this.constraints,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(constraints != null),
-    assert(constraints != null);
+       super(key: key, child: child) {
     assert(constraints.debugAssertIsValid());
   }
 
@@ -1028,17 +1014,16 @@ class FractionallySizedBox extends SingleChildRenderObjectWidget {
   ///
   /// If non-null, the [widthFactor] and [heightFactor] arguments must be
   /// non-negative.
-  FractionallySizedBox({
+  const FractionallySizedBox({
     Key key,
     this.alignment: FractionalOffset.center,
     this.widthFactor,
     this.heightFactor,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(alignment != null),
-    assert(alignment != null && alignment.dx != null && alignment.dy != null);
+       assert(widthFactor == null || widthFactor >= 0.0),
-    assert(widthFactor == null || widthFactor >= 0.0);
+       assert(heightFactor == null || heightFactor >= 0.0),
-    assert(heightFactor == null || heightFactor >= 0.0);
+       super(key: key, child: child);
-  }
 
   /// If non-null, the fraction of the incoming width given to the child.
   ///
@@ -1105,15 +1090,14 @@ class LimitedBox extends SingleChildRenderObjectWidget {
   ///
   /// The [maxWidth] and [maxHeight] arguments must not be null and must not be
   /// negative.
-  LimitedBox({
+  const LimitedBox({
     Key key,
     this.maxWidth: double.INFINITY,
     this.maxHeight: double.INFINITY,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(maxWidth != null && maxWidth >= 0.0),
-    assert(maxWidth != null && maxWidth >= 0.0);
+       assert(maxHeight != null && maxHeight >= 0.0),
-    assert(maxHeight != null && maxHeight >= 0.0);
+       super(key: key, child: child);
-  }
 
   /// The maximum width limit to apply in the absence of a maxWidth constraint.
   final double maxWidth;
@@ -1150,7 +1134,7 @@ class LimitedBox extends SingleChildRenderObjectWidget {
 /// See [RenderOverflowBox] for details.
 class OverflowBox extends SingleChildRenderObjectWidget {
   /// Creates a widget that lets its child overflow itself.
-  OverflowBox({
+  const OverflowBox({
     Key key,
     this.alignment: FractionalOffset.center,
     this.minWidth,
@@ -1227,15 +1211,14 @@ class SizedOverflowBox extends SingleChildRenderObjectWidget {
   /// Creates a widget of a given size that lets its child overflow.
   ///
   /// The [size] argument must not be null.
-  SizedOverflowBox({
+  const SizedOverflowBox({
     Key key,
     @required this.size,
     this.alignment: FractionalOffset.center,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(size != null),
-    assert(size != null);
+       assert(alignment != null),
-    assert(alignment != null && alignment.dx != null && alignment.dy != null);
+       super(key: key, child: child);
-  }
 
   /// How to align the child.
   ///
@@ -1280,10 +1263,9 @@ class SizedOverflowBox extends SingleChildRenderObjectWidget {
 /// room in the parent.
 class Offstage extends SingleChildRenderObjectWidget {
   /// Creates a widget that visually hides its child.
-  Offstage({ Key key, this.offstage: true, Widget child })
+  const Offstage({ Key key, this.offstage: true, Widget child })
-    : super(key: key, child: child) {
+    : assert(offstage != null),
-    assert(offstage != null);
+      super(key: key, child: child);
-  }
 
   /// Whether the child is hidden from the rest of the tree.
   ///
@@ -1355,13 +1337,12 @@ class AspectRatio extends SingleChildRenderObjectWidget {
   /// Creates a widget with a specific aspect ratio.
   ///
   /// The [aspectRatio] argument must not be null.
-  AspectRatio({
+  const AspectRatio({
     Key key,
     @required this.aspectRatio,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(aspectRatio != null),
-    assert(aspectRatio != null);
+       super(key: key, child: child);
-  }
 
   /// The aspect ratio to attempt to use.
   ///
@@ -1400,7 +1381,7 @@ class IntrinsicWidth extends SingleChildRenderObjectWidget {
   /// Creates a widget that sizes its child to the child's intrinsic width.
   ///
   /// This class is relatively expensive. Avoid using it where possible.
-  IntrinsicWidth({ Key key, this.stepWidth, this.stepHeight, Widget child })
+  const IntrinsicWidth({ Key key, this.stepWidth, this.stepHeight, Widget child })
     : super(key: key, child: child);
 
   /// If non-null, force the child's width to be a multiple of this value.
@@ -1431,7 +1412,7 @@ class IntrinsicHeight extends SingleChildRenderObjectWidget {
   /// Creates a widget that sizes its child to the child's intrinsic height.
   ///
   /// This class is relatively expensive. Avoid using it where possible.
-  IntrinsicHeight({ Key key, Widget child }) : super(key: key, child: child);
+  const IntrinsicHeight({ Key key, Widget child }) : super(key: key, child: child);
 
   @override
   RenderIntrinsicHeight createRenderObject(BuildContext context) => new RenderIntrinsicHeight();
@@ -1449,15 +1430,14 @@ class Baseline extends SingleChildRenderObjectWidget {
   /// Creates a widget that positions its child according to the child's baseline.
   ///
   /// The [baseline] and [baselineType] arguments must not be null.
-  Baseline({
+  const Baseline({
     Key key,
     @required this.baseline,
     @required this.baselineType,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(baseline != null),
-    assert(baseline != null);
+       assert(baselineType != null),
-    assert(baselineType != null);
+       super(key: key, child: child);
-  }
 
   /// The number of logical pixels from the top of this box at which to position
   /// the child's baseline.
@@ -1502,7 +1482,7 @@ class Baseline extends SingleChildRenderObjectWidget {
 ///  * [SliverGrid], which displays multiple box widgets in arbitrary positions.
 class SliverToBoxAdapter extends SingleChildRenderObjectWidget {
   /// Creates a sliver that contains a single box widget.
-  SliverToBoxAdapter({
+  const SliverToBoxAdapter({
     Key key,
     Widget child,
   }) : super(key: key, child: child);
@@ -1531,13 +1511,12 @@ class SliverPadding extends SingleChildRenderObjectWidget {
   /// Creates a sliver that applies padding on each side of another sliver.
   ///
   /// The [padding] argument must not be null.
-  SliverPadding({
+  const SliverPadding({
     Key key,
     @required this.padding,
     Widget sliver,
-  }) : super(key: key, child: sliver) {
+  }) : assert(padding != null),
-    assert(padding != null);
+       super(key: key, child: sliver);
-  }
 
   /// The amount of space by which to inset the child sliver.
   final EdgeInsets padding;
@@ -1576,9 +1555,8 @@ class BlockBody extends MultiChildRenderObjectWidget {
     Key key,
     this.mainAxis: Axis.vertical,
     List<Widget> children: const <Widget>[],
-  }) : super(key: key, children: children) {
+  }) : assert(mainAxis != null),
-    assert(mainAxis != null);
+       super(key: key, children: children);
-  }
 
   /// The direction to use as the main axis.
   final Axis mainAxis;
@@ -1730,7 +1708,7 @@ class Positioned extends ParentDataWidget<Stack> {
   /// [width]), and only two out of the three vertical values ([top],
   /// [bottom], [height]), can be set. In each case, at least one of
   /// the three must be null.
-  Positioned({
+  const Positioned({
     Key key,
     this.left,
     this.top,
@@ -1739,10 +1717,9 @@ class Positioned extends ParentDataWidget<Stack> {
     this.width,
     this.height,
     @required Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(left == null || right == null || width == null),
-    assert(left == null || right == null || width == null);
+       assert(top == null || bottom == null || height == null),
-    assert(top == null || bottom == null || height == null);
+       super(key: key, child: child);
-  }
 
   /// Creates a Positioned object with the values from the given [Rect].
   ///
@@ -1956,12 +1933,12 @@ class Flex extends MultiChildRenderObjectWidget {
     this.crossAxisAlignment: CrossAxisAlignment.center,
     this.textBaseline,
     List<Widget> children: const <Widget>[],
-  }) : super(key: key, children: children) {
+  }) : assert(direction != null),
-    assert(direction != null);
+       assert(mainAxisAlignment != null),
-    assert(mainAxisAlignment != null);
+       assert(mainAxisSize != null),
-    assert(mainAxisSize != null);
+       assert(crossAxisAlignment != null),
-    assert(crossAxisAlignment != null);
+       super(key: key, children: children) {
-    assert(crossAxisAlignment != CrossAxisAlignment.baseline || textBaseline != null);
+    assert(crossAxisAlignment != CrossAxisAlignment.baseline || textBaseline != null); // https://github.com/dart-lang/sdk/issues/29278
   }
 
   /// The direction to use as the main axis.
@@ -2185,7 +2162,7 @@ class Column extends Flex {
 class Flexible extends ParentDataWidget<Flex> {
   /// Creates a widget that controls how a child of a [Row], [Column], or [Flex]
   /// flexes.
-  Flexible({
+  const Flexible({
     Key key,
     this.flex: 1,
     this.fit: FlexFit.loose,
@@ -2257,7 +2234,7 @@ class Flexible extends ParentDataWidget<Flex> {
 class Expanded extends Flexible {
   /// Creates a widget that expands a child of a [Row], [Column], or [Flex]
   /// expand to fill the available space in the main axis.
-  Expanded({
+  const Expanded({
     Key key,
     int flex: 1,
     @required Widget child,
@@ -2419,9 +2396,9 @@ class Flow extends MultiChildRenderObjectWidget {
     Key key,
     @required this.delegate,
     List<Widget> children: const <Widget>[],
-  }) : super(key: key, children: RepaintBoundary.wrapAll(children)) {
+  }) : assert(delegate != null),
-    assert(delegate != null);
+       super(key: key, children: RepaintBoundary.wrapAll(children));
-  }
+       // https://github.com/dart-lang/sdk/issues/29277
 
   /// Creates a flow layout.
   ///
@@ -2491,7 +2468,7 @@ class RichText extends LeafRenderObjectWidget {
   /// Creates a paragraph of rich text.
   ///
   /// The [text], [softWrap], and [overflow] arguments must not be null.
-  RichText({
+  const RichText({
     Key key,
     @required this.text,
     this.textAlign,
@@ -2499,12 +2476,11 @@ class RichText extends LeafRenderObjectWidget {
     this.overflow: TextOverflow.clip,
     this.textScaleFactor: 1.0,
     this.maxLines,
-  }) : super(key: key) {
+  }) : assert(text != null),
-    assert(text != null);
+       assert(softWrap != null),
-    assert(softWrap != null);
+       assert(overflow != null),
-    assert(overflow != null);
+       assert(textScaleFactor != null),
-    assert(textScaleFactor != null);
+       super(key: key);
-  }
 
   /// The text to display in this widget.
   final TextSpan text;
@@ -2564,7 +2540,7 @@ class RawImage extends LeafRenderObjectWidget {
   /// Creates a widget that displays an image.
   ///
   /// The [scale] and [repeat] arguments must not be null.
-  RawImage({
+  const RawImage({
     Key key,
     this.image,
     this.width,
@@ -2575,10 +2551,9 @@ class RawImage extends LeafRenderObjectWidget {
     this.alignment,
     this.repeat: ImageRepeat.noRepeat,
     this.centerSlice
-  }) : super(key: key) {
+  }) : assert(scale != null),
-    assert(scale != null);
+       assert(repeat != null),
-    assert(repeat != null);
+       super(key: key);
-  }
 
   /// The image to display.
   final ui.Image image;
@@ -2686,14 +2661,13 @@ class DefaultAssetBundle extends InheritedWidget {
   /// Creates a widget that determines the default asset bundle for its descendants.
   ///
   /// The [bundle] and [child] arguments must not be null.
-  DefaultAssetBundle({
+  const DefaultAssetBundle({
     Key key,
     @required this.bundle,
     @required Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(bundle != null),
-    assert(bundle != null);
+       assert(child != null),
-    assert(child != null);
+       super(key: key, child: child);
-  }
 
   /// The bundle to use as a default.
   final AssetBundle bundle;
@@ -2729,15 +2703,13 @@ class WidgetToRenderBoxAdapter extends LeafRenderObjectWidget {
   /// The [renderBox] argument must not be null.
   WidgetToRenderBoxAdapter({
     @required this.renderBox,
-    this.onBuild
+    this.onBuild,
-  }) :
+  }) : assert(renderBox != null),
        // WidgetToRenderBoxAdapter objects are keyed to their render box. This
        // prevents the widget being used in the widget hierarchy in two different
        // places, which would cause the RenderBox to get inserted in multiple
        // places in the RenderObject tree.
-       super(key: new GlobalObjectKey(renderBox)) {
+       super(key: new GlobalObjectKey(renderBox));
-    assert(renderBox != null);
-  }
 
   /// The render box to place in the widget tree.
   final RenderBox renderBox;
@@ -2772,7 +2744,7 @@ class Listener extends SingleChildRenderObjectWidget {
   /// Creates a widget that forwards point events to callbacks.
   ///
   /// The [behavior] argument defaults to [HitTestBehavior.deferToChild].
-  Listener({
+  const Listener({
     Key key,
     this.onPointerDown,
     this.onPointerMove,
@@ -2780,9 +2752,8 @@ class Listener extends SingleChildRenderObjectWidget {
     this.onPointerCancel,
     this.behavior: HitTestBehavior.deferToChild,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(behavior != null),
-    assert(behavior != null);
+       super(key: key, child: child);
-  }
 
   /// Called when a pointer comes into contact with the screen at this object.
   final PointerDownEventListener onPointerDown;
@@ -2858,7 +2829,7 @@ class Listener extends SingleChildRenderObjectWidget {
 /// for the surround tree.
 class RepaintBoundary extends SingleChildRenderObjectWidget {
   /// Creates a widget that isolates repaints.
-  RepaintBoundary({ Key key, Widget child }) : super(key: key, child: child);
+  const RepaintBoundary({ Key key, Widget child }) : super(key: key, child: child);
 
   /// Wraps the given child in a [RepaintBoundary].
   ///
@@ -2906,14 +2877,13 @@ class IgnorePointer extends SingleChildRenderObjectWidget {
   ///
   /// The [ignoring] argument must not be null. If [ignoringSemantics], this
   /// render object will be ignored for semantics if [ignoring] is true.
-  IgnorePointer({
+  const IgnorePointer({
     Key key,
     this.ignoring: true,
     this.ignoringSemantics,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(ignoring != null),
-    assert(ignoring != null);
+       super(key: key, child: child);
-  }
 
   /// Whether this widget is ignored during hit testing.
   ///
@@ -2966,13 +2936,12 @@ class AbsorbPointer extends SingleChildRenderObjectWidget {
   /// Creates a widget that absorbs pointers during hit testing.
   ///
   /// The [absorbing] argument must not be null
-  AbsorbPointer({
+  const AbsorbPointer({
     Key key,
     this.absorbing: true,
     Widget child
-  }) : super(key: key, child: child) {
+  }) : assert(absorbing != null),
-    assert(absorbing != null);
+       super(key: key, child: child);
-  }
 
   /// Whether this widget absorbs pointers during hit testing.
   ///
@@ -3000,7 +2969,7 @@ class MetaData extends SingleChildRenderObjectWidget {
   /// Creates a widget that hold opaque meta data.
   ///
   /// The [behavior] argument defaults to [HitTestBehavior.deferToChild].
-  MetaData({
+  const MetaData({
     Key key,
     this.metaData,
     this.behavior: HitTestBehavior.deferToChild,
@@ -3061,15 +3030,14 @@ class Semantics extends SingleChildRenderObjectWidget {
   /// Creates a semantic annotation.
   ///
   /// The [container] argument must not be null.
-  Semantics({
+  const Semantics({
     Key key,
     Widget child,
     this.container: false,
     this.checked,
     this.label
-  }) : super(key: key, child: child) {
+  }) : assert(container != null),
-    assert(container != null);
+       super(key: key, child: child);
-  }
 
   /// If 'container' is true, this Widget will introduce a new node in
   /// the semantics tree. Otherwise, the semantics will be merged with
@@ -3138,7 +3106,7 @@ class Semantics extends SingleChildRenderObjectWidget {
 /// callbacks.
 class MergeSemantics extends SingleChildRenderObjectWidget {
   /// Creates a widget that merges the semantics of its descendants.
-  MergeSemantics({ Key key, Widget child }) : super(key: key, child: child);
+  const MergeSemantics({ Key key, Widget child }) : super(key: key, child: child);
 
   @override
   RenderMergeSemantics createRenderObject(BuildContext context) => new RenderMergeSemantics();
@@ -3152,7 +3120,7 @@ class MergeSemantics extends SingleChildRenderObjectWidget {
 /// redundant with the chip label.
 class ExcludeSemantics extends SingleChildRenderObjectWidget {
   /// Creates a widget that drops all the semantics of its descendants.
-  ExcludeSemantics({ Key key, Widget child }) : super(key: key, child: child);
+  const ExcludeSemantics({ Key key, Widget child }) : super(key: key, child: child);
 
   @override
   RenderExcludeSemantics createRenderObject(BuildContext context) => new RenderExcludeSemantics();
@@ -3163,12 +3131,11 @@ class ExcludeSemantics extends SingleChildRenderObjectWidget {
 /// Useful for attaching a key to an existing widget.
 class KeyedSubtree extends StatelessWidget {
   /// Creates a widget that builds its child.
-  KeyedSubtree({
+  const KeyedSubtree({
     Key key,
     @required this.child
-  }) : super(key: key) {
+  }) : assert(child != null),
-    assert(child != null);
+       super(key: key);
-  }
 
   /// The widget below this widget in the tree.
   final Widget child;
@@ -3209,12 +3176,11 @@ class Builder extends StatelessWidget {
   /// Creates a widget that delegates its build to a callback.
   ///
   /// The [builder] argument must not be null.
-  Builder({
+  const Builder({
     Key key,
     @required this.builder
-  }) : super(key: key) {
+  }) : assert(builder != null),
-    assert(builder != null);
+       super(key: key);
-  }
 
   /// Called to obtain the child widget.
   ///
@@ -3243,12 +3209,11 @@ class StatefulBuilder extends StatefulWidget {
   /// Creates a widget that both has state and delegates its build to a callback.
   ///
   /// The [builder] argument must not be null.
-  StatefulBuilder({
+  const StatefulBuilder({
     Key key,
     @required this.builder
-  }) : super(key: key) {
+  }) : assert(builder != null),
-    assert(builder != null);
+       super(key: key);
-  }
 
   /// Called to obtain the child widget.
   ///

packages/flutter/lib/src/widgets/framework.dart

@@ -1339,9 +1339,9 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
   /// The [children] argument must not be null and must not contain any null
   /// objects.
   MultiChildRenderObjectWidget({ Key key, this.children: const <Widget>[] })
-    : super(key: key) {
+    : assert(children != null),
-    assert(children != null);
+      super(key: key) {
-    assert(!children.any((Widget child) => child == null));
+    assert(!children.any((Widget child) => child == null)); // https://github.com/dart-lang/sdk/issues/29276
   }
 
   /// The widgets below this widget in the tree.