use symbol literals instead of const ctor in packages

pkg/polymer_expressions/lib/polymer_expressions.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A binding delegate used with Polymer elements that
  * allows for complex binding expressions, including
  * property access, function invocation,
  * list/map indexing, and two-way filtering.
  *
@@ skipping 72 matching lines @@
 
   getInstanceModel(Element template, model) {
     if (model is! Scope) {
       var _scope = new Scope(model: model, variables: globals);
       return _scope;
     }
     return model;
   }
 }
 
-class _Binding extends Object with ChangeNotifierMixin {
+class _Binding extends ChangeNotifierBase {
   final Scope _scope;
   final ExpressionObserver _expr;
   final _converter;
   var _value;
 
   _Binding(Expression expr, Scope scope, [this._converter])
       : _expr = observe(expr, scope),
         _scope = scope {
     _expr.onUpdate.listen(_setValue).onError((e) {
       _logger.warning("Error evaluating expression '$_expr': ${e.message}");
@@ skipping 26 matching lines @@
   get value => _value;
 
   set value(v) {
     try {
       assign(_expr, v, _scope);
     } on EvalException catch (e) {
       // silently swallow binding errors
     }
   }
 }