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

sky/sdk/lib/framework/rendering/box.dart

Index: sky/sdk/lib/framework/rendering/box.dart
diff --git a/sky/sdk/lib/framework/rendering/box.dart b/sky/sdk/lib/framework/rendering/box.dart
index c25b9f62aefcc8c8156fa85e36c4b06f1eac6085..53021807e804c5eb4ebaef53ee673336ccfeb4f7 100644
--- a/sky/sdk/lib/framework/rendering/box.dart
+++ b/sky/sdk/lib/framework/rendering/box.dart
@@ -216,7 +216,7 @@ abstract class RenderBox extends RenderObject {
     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);