Introduce BlockViewport.

sky/sdk/lib/rendering/box.dart

 // Copyright 2015 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 'dart:math' as math;
 import 'dart:sky' as sky;
 
 import 'package:vector_math/vector_math.dart';
 
 import '../base/debug.dart';
@@ skipping 402 matching lines @@
         // other than us, the only object that's allowed to read our size is our parent, if they're said they will
         // if you hit this assert trying to access a child's size, pass parentUsesSize: true in layout()
         assert(debugDoingThisResize || debugDoingThisLayout ||
                (RenderObject.debugActiveLayout == parent && _size._canBeUsedByParent));
       }
       assert(_size == this._size); // TODO(ianh): Remove this once the analyzer is cleverer
     }
     return _size;
   }
   void set size(Size value) {
-    assert(RenderObject.debugDoingLayout);
     assert((sizedByParent && debugDoingThisResize) ||
            (!sizedByParent && debugDoingThisLayout));
     if (value is _DebugSize) {
       assert(value._canBeUsedByParent);
       assert(value._owner.parent == this);
     }
     _size = inDebugBuild ? new _DebugSize(value, this, debugCanParentUseSize) : value;
   }
 
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}size: ${size}\n';
@@ skipping 1001 matching lines @@
 
   void defaultPaint(PaintingCanvas canvas, Offset offset) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position + offset);
       child = child.parentData.nextSibling;
     }
   }
 }