Make the menu items in the drawer change colour when tapped, per Material spec.

sky/sdk/lib/widgets/default_text_style.dart

Index: sky/sdk/lib/widgets/default_text_style.dart
diff --git a/sky/sdk/lib/widgets/default_text_style.dart b/sky/sdk/lib/widgets/default_text_style.dart
index a8cc5f0d14a2b43657bba8b01719493b0d3cebf0..ea64c22daedb596305bfd315304509792fd2f763 100644
--- a/sky/sdk/lib/widgets/default_text_style.dart
+++ b/sky/sdk/lib/widgets/default_text_style.dart
@@ -23,4 +23,7 @@ class DefaultTextStyle extends Inherited {
     DefaultTextStyle result = component.inheritedOfType(DefaultTextStyle);
     return result == null ? null : result.style;
   }
+
+  bool syncShouldNotify(DefaultTextStyle old) => style != old.style;
+
 }