Re-land "Add a Color class to dart:sky."

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

Index: sky/sdk/lib/framework/rendering/render_box.dart
diff --git a/sky/sdk/lib/framework/rendering/render_box.dart b/sky/sdk/lib/framework/rendering/render_box.dart
index a3d47124bd58482af1070b4122599aac22378384..dc081e1a159a7a21d7e1af6fa23f2e51bc3839a0 100644
--- a/sky/sdk/lib/framework/rendering/render_box.dart
+++ b/sky/sdk/lib/framework/rendering/render_box.dart
@@ -220,11 +220,13 @@ class RenderPadding extends RenderBox with RenderNodeWithChildMixin<RenderBox> {
 
 // This must be immutable, because we won't notice when it changes
 class BoxDecoration {
-  const BoxDecoration({
-    this.backgroundColor
-  });
+  // TODO(mpcomplete): go through and change the users of this class to pass
+  // a Color object.
+  BoxDecoration({
+    backgroundColor
+  }) : backgroundColor = new sky.Color(backgroundColor);
 
-  final int backgroundColor;
+  final sky.Color backgroundColor;
 }
 
 class RenderDecoratedBox extends RenderProxyBox {