use symbol literals instead of const ctor in packages

pkg/polymer_expressions/lib/polymer_expressions.dart

Index: pkg/polymer_expressions/lib/polymer_expressions.dart
diff --git a/pkg/polymer_expressions/lib/polymer_expressions.dart b/pkg/polymer_expressions/lib/polymer_expressions.dart
index 3cee82067c542d0d11497846e217746156900e14..ea02bbbe84f10de2e8ddc7294f381b0fd65891bc 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -90,7 +90,7 @@ class PolymerExpressions extends BindingDelegate {
   }
 }
 
-class _Binding extends Object with ChangeNotifierMixin {
+class _Binding extends ChangeNotifierBase {
   final Scope _scope;
   final ExpressionObserver _expr;
   final _converter;