More minor cleanup around overriding fields to narrow types.

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 '../framework/debug/utils.dart';
@@ skipping 242 matching lines @@
     _debugDoingThisLayout = false;
   }
   void layout(dynamic constraints, { bool parentUsesSize: false }) {
     _debugDoingThisLayout = true;
     _debugCanParentUseSize = parentUsesSize;
     super.layout(constraints, parentUsesSize: parentUsesSize);
     _debugCanParentUseSize = null;
     _debugDoingThisLayout = false;
   }
 
-  BoxConstraints get constraints { BoxConstraints result = super.constraints; return result; }
+  BoxConstraints get constraints => super.constraints;
   void performResize() {
     // default behaviour for subclasses that have sizedByParent = true
     size = constraints.constrain(Size.zero);
     assert(size.height < double.INFINITY);
     assert(size.width < double.INFINITY);
   }
   void performLayout() {
     // descendants have to either override performLayout() to set both
     // width and height and lay out children, or, set sizedByParent to
     // true so that performResize()'s logic above does its thing.
@@ skipping 769 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;
     }
   }
 }