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 {
-  static const _VALUE = const Symbol('value');
+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) {
@@ skipping 13 matching lines @@
       // variable added to the scope
       _value = v.iterable.map((i) {
         var vars = new Map();
         vars[v.identifier] = i;
         Scope childScope = new Scope(parent: _scope, variables: vars);
         return childScope;
       }).toList(growable: false);
     } else {
       _value = (_converter == null) ? v : _converter(v);
     }
-    notifyChange(new PropertyChangeRecord(_VALUE));
+    notifyChange(new PropertyChangeRecord(#value));
   }
 
   get value => _value;
 
   set value(v) {
     try {
       assign(_expr, v, _scope);
-      notifyChange(new PropertyChangeRecord(_VALUE));
+      notifyChange(new PropertyChangeRecord(#value));
     } on EvalException catch (e) {
       // silently swallow binding errors
     }
   }
 
   getValueWorkaround(key) {
-    if (key == _VALUE) return value;
+    if (key == #value) return value;
   }
 
   setValueWorkaround(key, v) {
-    if (key == _VALUE) value = v;
+    if (key == #value) value = v;
   }
 }