Short-circuit the relayoutSubtreeRoot when the child couldn't change dimensions anyway because the …

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 45 matching lines @@
     int value = 373;
     value = 37 * value + top.hashCode;
     value = 37 * value + left.hashCode;
     value = 37 * value + bottom.hashCode;
     value = 37 * value + right.hashCode;
     return value;
   }
   String toString() => "EdgeDims($top, $right, $bottom, $left)";
 }
 
-class BoxConstraints {
+class BoxConstraints extends Constraints {
   const BoxConstraints({
     this.minWidth: 0.0,
     this.maxWidth: double.INFINITY,
     this.minHeight: 0.0,
     this.maxHeight: double.INFINITY
   });
 
   BoxConstraints.tight(Size size)
     : minWidth = size.width,
       maxWidth = size.width,
@@ skipping 101 matching lines @@
 
   Size constrain(Size size) {
     Size result = new Size(constrainWidth(size.width), constrainHeight(size.height));
     if (size is _DebugSize)
       result = new _DebugSize(result, size._owner, size._canBeUsedByParent);
     return result;
   }
 
   bool get isInfinite => maxWidth >= double.INFINITY && maxHeight >= double.INFINITY;
 
-  bool get hasTightWidth => minWidth == maxWidth;
-  bool get hasTightHeight => minHeight == maxHeight;
+  bool get hasTightWidth => minWidth >= maxWidth;
+  bool get hasTightHeight => minHeight >= maxHeight;
   bool get isTight => hasTightWidth && hasTightHeight;
 
   bool operator ==(other) {
     if (identical(this, other))
       return true;
     return other is BoxConstraints &&
            minWidth == other.minWidth &&
            maxWidth == other.maxWidth &&
            minHeight == other.minHeight &&
            maxHeight == other.maxHeight;
@@ skipping 905 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }