Initial implementation of SnackBar

sky/sdk/lib/rendering/box.dart

Index: sky/sdk/lib/rendering/box.dart
diff --git a/sky/sdk/lib/rendering/box.dart b/sky/sdk/lib/rendering/box.dart
index aac7bbea44ca32bef32d1fd7e994195d282a3d56..78ec1d2c4d75075e52810734de486bb14c51c378 100644
--- a/sky/sdk/lib/rendering/box.dart
+++ b/sky/sdk/lib/rendering/box.dart
@@ -411,6 +411,7 @@ abstract class RenderBox extends RenderObject {
       if (RenderObject.debugActiveLayout != null) {
         // we are always allowed to access our own size (for print debugging and asserts if nothing else)
         // 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));
       }