Update BoxDecoration and RenderParagraph to use sky.Color instead of int.

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 f6bb40bf0484012044dd904d444cf13b4650eb8a..160edd76ad5530542d6d42fa2053432ae7dc0d33 100644
--- a/sky/sdk/lib/framework/rendering/box.dart
+++ b/sky/sdk/lib/framework/rendering/box.dart
@@ -251,11 +251,7 @@ class RenderPadding extends RenderBox with RenderNodeWithChildMixin<RenderBox> {
 
 // This must be immutable, because we won't notice when it changes
 class BoxDecoration {
-  // TODO(mpcomplete): go through and change the users of this class to pass
-  // a Color object.
-  BoxDecoration({
-    backgroundColor
-  }) : backgroundColor = new sky.Color(backgroundColor);
+  const BoxDecoration({this.backgroundColor});
 
   final sky.Color backgroundColor;
 }