Add Spacer Widget (#16642)

This adds a simple spacer widget first suggested by Scott Stoll (@ScottS2017) in the abandoned PR #15802

This PR replaces that one, cleans it up, and adds a test.

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

@@ -3233,6 +3233,7 @@ class PositionedDirectional extends StatelessWidget {
 ///  * [Column], for a version of this widget that is always vertical.
 ///  * [Expanded], to indicate children that should take all the remaining room.
 ///  * [Flexible], to indicate children that should share the remaining room but
+///  * [Spacer], a widget that takes up space proportional to it's flex value.
 ///    that may be sized smaller (leaving some remaining room unused).
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class Flex extends MultiChildRenderObjectWidget {
@@ -3565,6 +3566,7 @@ class Flex extends MultiChildRenderObjectWidget {
 ///  * [Expanded], to indicate children that should take all the remaining room.
 ///  * [Flexible], to indicate children that should share the remaining room but
 ///    that may by sized smaller (leaving some remaining room unused).
+///  * [Spacer], a widget that takes up space proportional to it's flex value.
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class Row extends Flex {
   /// Creates a horizontal array of children.
@@ -3759,6 +3761,7 @@ class Row extends Flex {
 ///    that may size smaller (leaving some remaining room unused).
 ///  * [SingleChildScrollView], whose documentation discusses some ways to
 ///    use a [Column] inside a scrolling container.
+///  * [Spacer], a widget that takes up space proportional to it's flex value.
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class Column extends Flex {
   /// Creates a vertical array of children.
@@ -3810,6 +3813,7 @@ class Column extends Flex {
 /// See also:
 ///
 ///  * [Expanded], which forces the child to expand to fill the available space.
+///  * [Spacer], a widget that takes up space proportional to it's flex value.
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class Flexible extends ParentDataWidget<Flex> {
   /// Creates a widget that controls how a child of a [Row], [Column], or [Flex]
@@ -3883,6 +3887,7 @@ class Flexible extends ParentDataWidget<Flex> {
 /// See also:
 ///
 ///  * [Flexible], which does not force the child to fill the available space.
+///  * [Spacer], a widget that takes up space proportional to it's flex value.
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
 class Expanded extends Flexible {
   /// Creates a widget that expands a child of a [Row], [Column], or [Flex]

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

@@ -0,0 +1,67 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/rendering.dart';
+
+import 'basic.dart';
+import 'framework.dart';
+
+/// Spacer creates an adjustable, empty spacer that can be used to tune the
+/// spacing between widgets in a [Flex] container, like [Row] or [Column].
+///
+/// The [Spacer] widget will take up any available space, so setting the
+/// [Flex.mainAxisAlignment] on a flex container that contains a [Spacer] to
+/// [MainAxisAlignment.spaceAround], [MainAxisAlignment.spaceBetween], or
+/// [MainAxisAlignment.spaceEvenly] will not have any visible effect: the
+/// [Spacer] has taken up all of the additional space, so there is none left to
+/// redistribute.
+///
+/// ## Sample code
+///
+/// ```dart
+/// new Row(
+///   children: <Widget>[
+///     new Text('Begin'),
+///     new Spacer(), // Defaults to a flex of one.
+///     new Text('Middle'),
+///     // Gives twice the space between Middle and End than Begin and Middle.
+///     new Spacer(flex: 2),
+///     new Text('End'),
+///   ],
+/// )
+/// ```
+///
+/// See also:
+///
+///  * [Row] and [Column], which are the most common containers to use a Spacer
+///    in.
+///  * [SizedBox], to create a box with a specific size and an optional child.
+class Spacer extends StatelessWidget {
+  /// Creates a flexible space to insert into a [Flexible] widget.
+  ///
+  /// The [flex] parameter may not be null or less than one.
+  const Spacer({Key key, this.flex: 1})
+      : assert(flex != null),
+        assert(flex > 0),
+        super(key: key);
+
+  /// The flex factor to use in determining how much space to take up.
+  ///
+  /// The amount of space the [Spacer] can occupy in the main axis is determined
+  /// by dividing the free space proportionately, after placing the inflexible
+  /// children, according to the flex factors of the flexible children.
+  ///
+  /// Defaults to one.
+  final int flex;
+
+  @override
+  Widget build(BuildContext context) {
+    return new Expanded(
+      flex: flex,
+      child: new ConstrainedBox(
+        constraints: const BoxConstraints.expand(),
+      ),
+    );
+  }
+}

packages/flutter/lib/widgets.dart

@@ -86,6 +86,7 @@ export 'src/widgets/size_changed_layout_notifier.dart';
 export 'src/widgets/sliver.dart';
 export 'src/widgets/sliver_persistent_header.dart';
 export 'src/widgets/sliver_prototype_extent_list.dart';
+export 'src/widgets/spacer.dart';
 export 'src/widgets/status_transitions.dart';
 export 'src/widgets/table.dart';
 export 'src/widgets/text.dart';

packages/flutter/test/widgets/spacer_test.dart

@@ -0,0 +1,55 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('Spacer takes up space.', (WidgetTester tester) async {
+    await tester.pumpWidget(new Column(
+      children: const <Widget>[
+        const SizedBox(width: 10.0, height: 10.0),
+        const Spacer(),
+        const SizedBox(width: 10.0, height: 10.0),
+      ],
+    ));
+    final Rect spacerRect = tester.getRect(find.byType(Spacer));
+    expect(spacerRect.size, const Size(800.0, 580.0));
+    expect(spacerRect.topLeft, const Offset(0.0, 10.0));
+  });
+
+  testWidgets('Spacer takes up space proportional to flex.', (WidgetTester tester) async {
+    const Spacer spacer1 = const Spacer();
+    const Spacer spacer2 = const Spacer(flex: 1);
+    const Spacer spacer3 = const Spacer(flex: 2);
+    const Spacer spacer4 = const Spacer(flex: 4);
+    await tester.pumpWidget(new Row(
+      textDirection: TextDirection.rtl,
+      children: const <Widget>[
+        const SizedBox(width: 10.0, height: 10.0),
+        spacer1,
+        const SizedBox(width: 10.0, height: 10.0),
+        spacer2,
+        const SizedBox(width: 10.0, height: 10.0),
+        spacer3,
+        const SizedBox(width: 10.0, height: 10.0),
+        spacer4,
+        const SizedBox(width: 10.0, height: 10.0),
+      ],
+    ));
+    final Rect spacer1Rect = tester.getRect(find.byType(Spacer).at(0));
+    final Rect spacer2Rect = tester.getRect(find.byType(Spacer).at(1));
+    final Rect spacer3Rect = tester.getRect(find.byType(Spacer).at(2));
+    final Rect spacer4Rect = tester.getRect(find.byType(Spacer).at(3));
+    expect(spacer1Rect.size.height, 600.0);
+    expect(spacer1Rect.size.width, closeTo(93.8, 0.1));
+    expect(spacer1Rect.left, closeTo(696.3, 0.1));
+    expect(spacer2Rect.size.width, closeTo(93.8, 0.1));
+    expect(spacer2Rect.left, closeTo(592.5, 0.1));
+    expect(spacer3Rect.size.width, spacer2Rect.size.width * 2.0);
+    expect(spacer3Rect.left, closeTo(395.0, 0.1));
+    expect(spacer4Rect.size.width, spacer3Rect.size.width * 2.0);
+    expect(spacer4Rect.left, closeTo(10.0, 0.1));
+  });
+}