Let's hide double.INFINITY a bit more, by providing cleaner APIs for the cases where we're currentl…

sky/sdk/lib/widgets/basic.dart

Index: sky/sdk/lib/widgets/basic.dart
diff --git a/sky/sdk/lib/widgets/basic.dart b/sky/sdk/lib/widgets/basic.dart
index 3959af38bdbeab3f6940257892229eb6750a25cd..7c9f8d4533f04dec13f53883eb069bbcb2ea4fb2 100644
--- a/sky/sdk/lib/widgets/basic.dart
+++ b/sky/sdk/lib/widgets/basic.dart
@@ -290,10 +290,7 @@ class Container extends Component {
     Widget current = child;
 
     if (child == null && width == null && height == null)
-      current = new SizedBox(
-        width: double.INFINITY,
-        height: double.INFINITY
-      );
+      current = new ConstrainedBox(constraints: BoxConstraints.expand);
 
     if (padding != null)
       current = new Padding(padding: padding, child: current);