diff --git a/packages/flutter/lib/rendering/block.dart b/packages/flutter/lib/rendering/block.dart
index d5f3497e34..7214c533d7 100644
--- a/packages/flutter/lib/rendering/block.dart
+++ b/packages/flutter/lib/rendering/block.dart
@@ -49,6 +49,16 @@ abstract class RenderBlockBase extends RenderBox with ContainerRenderObjectMixin
     return new BoxConstraints.tightFor(height: constraints.constrainHeight(constraints.maxHeight));
   }
 
+  double get _mainAxisExtent {
+    RenderBox child = lastChild;
+    if (child == null)
+      return 0.0;
+    BoxParentData parentData = child.parentData;
+    return _isVertical ?
+        parentData.position.y + child.size.height :
+        parentData.position.x + child.size.width;
+  }
+
   void performLayout() {
     BoxConstraints innerConstraints = _getInnerConstraints(constraints);
     double position = 0.0;
@@ -60,6 +70,10 @@ abstract class RenderBlockBase extends RenderBox with ContainerRenderObjectMixin
       position += _isVertical ? child.size.height : child.size.width;
       child = child.parentData.nextSibling;
     }
+    size = _isVertical ?
+        constraints.constrain(new Size(constraints.maxWidth, _mainAxisExtent)) :
+        constraints.constrain(new Size(_mainAxisExtent, constraints.maxHeight));
+    assert(!size.isInfinite);
   }
 
 }
@@ -137,23 +151,11 @@ class RenderBlock extends RenderBlockBase {
     return defaultComputeDistanceToFirstActualBaseline(baseline);
   }
 
-  double get _mainAxisExtent {
-    RenderBox child = lastChild;
-    if (child == null)
-      return 0.0;
-    BoxParentData parentData = child.parentData;
-    return _isVertical ?
-        parentData.position.y + child.size.height :
-        parentData.position.x + child.size.width;
-  }
-
   void performLayout() {
-    assert(_isVertical ? constraints.maxHeight >= double.INFINITY : constraints.maxWidth >= double.INFINITY);
+    assert((_isVertical ? constraints.maxHeight >= double.INFINITY : constraints.maxWidth >= double.INFINITY) &&
+           'RenderBlock does not clip or resize its children, so it must be placed in a parent that does not constrain ' +
+           'the block\'s main direction. You probably want to put the RenderBlock inside a RenderViewport.' is String);
     super.performLayout();
-    size = _isVertical ?
-        constraints.constrain(new Size(constraints.maxWidth, _mainAxisExtent)) :
-        constraints.constrain(new Size(_mainAxisExtent, constraints.maxHeight));
-    assert(!size.isInfinite);
   }
 
   void paint(PaintingContext context, Offset offset) {
@@ -168,9 +170,6 @@ class RenderBlock extends RenderBlockBase {
 
 class RenderBlockViewport extends RenderBlockBase {
 
-  // sizes itself to the given constraints
-  // at the start of layout, calls callback
-
   RenderBlockViewport({
     LayoutCallback callback,
     List<RenderBox> children,
@@ -200,20 +199,30 @@ class RenderBlockViewport extends RenderBlockBase {
       markNeedsPaint();
   }
 
+  double _noIntrinsicDimensions() {
+    assert(() {
+      'RenderBlockViewport does not support returning intrinsic dimensions. ' +
+      'Calculating the intrinsic dimensions would require walking the entire child list, ' +
+      'which defeats the entire point of having a lazily-built list of children.';
+      return false;
+    });
+    return 0.0;
+  }
+
   double getMinIntrinsicWidth(BoxConstraints constraints) {
-    return constraints.constrainWidth();
+    return _noIntrinsicDimensions();
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
-    return constraints.constrainWidth();
+    return _noIntrinsicDimensions();
   }
 
   double getMinIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight();
+    return _noIntrinsicDimensions();
   }
 
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight();
+    return _noIntrinsicDimensions();
   }
 
   // We don't override computeDistanceToActualBaseline(), because we
@@ -221,16 +230,8 @@ class RenderBlockViewport extends RenderBlockBase {
   // scroll the RenderBlockViewport, it would shift in its parent if
   // the parent was baseline-aligned, which makes no sense.
 
-  bool get sizedByParent => true;
-
-  void performResize() {
-    size = constraints.biggest;
-    assert(!size.isInfinite);
-  }
-
   bool get debugDoesLayoutWithCallback => true;
   void performLayout() {
-    assert(constraints.maxHeight < double.INFINITY);
     if (_callback != null) {
       try {
         _inCallback = true;
diff --git a/packages/flutter/lib/widgets/mixed_viewport.dart b/packages/flutter/lib/widgets/mixed_viewport.dart
index 3b8ce4f1f9..c8e58c486a 100644
--- a/packages/flutter/lib/widgets/mixed_viewport.dart
+++ b/packages/flutter/lib/widgets/mixed_viewport.dart
@@ -78,7 +78,7 @@ class MixedViewportLayoutState {
 }
 
 class MixedViewport extends RenderObjectWrapper {
-  MixedViewport({ this.builder, this.startOffset, this.token, this.layoutState, Key key })
+  MixedViewport({ Key key, this.builder, this.startOffset, this.token, this.layoutState })
     : super(key: key) {
     assert(this.layoutState != null);
   }
@@ -258,7 +258,11 @@ class MixedViewport extends RenderObjectWrapper {
 
     final List<double> offsets = layoutState._childOffsets;
     final Map<_Key, Widget> childrenByKey = layoutState._childrenByKey;
-    final double height = renderObject.size.height;
+    final double height = constraints.maxHeight;
+    assert(height < double.INFINITY &&
+      'There is no point putting a lazily-built MixedViewport inside a box with infinite internal height ' +
+      '(e.g. inside something that scrolls), because it would then just eagerly build all the children. ' +
+      'You probably want to put the MixedViewport inside a container with a fixed height.' is String);
     final double endOffset = startOffset + height;
     BoxConstraints innerConstraints = new BoxConstraints.tightFor(width: constraints.constrainWidth());