Replace some 'as' operators with equivalent code that doesn't do any checks in production mode.

sky/sdk/lib/framework/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 'dart:typed_data';
 import 'object.dart';
 import '../painting/shadows.dart';
 import 'package:vector_math/vector_math.dart';
@@ skipping 198 matching lines @@
 
   // getMaxIntrinsicHeight should return the smallest height beyond which
   // increasing the height never decreases the width.
   // If the layout algorithm used is width-in-height-out, i.e. the height
   // depends on the width and not vice versa, then this will return the same
   // as getMinIntrinsicHeight().
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
     return constraints.constrainHeight(0.0);
   }
 
-  BoxConstraints get constraints => super.constraints as BoxConstraints;
+  BoxConstraints get constraints { BoxConstraints result = super.constraints; return result; }
   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 889 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;
     }
   }
 }