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 ddb85fe667a7ff15401e582f9994c561c67f50b5..c84562d0a7240f579cc08a404e883c43ce6e7127 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -90,8 +90,7 @@ class PolymerExpressions extends BindingDelegate {
   }
 }
 
-class _Binding extends Object with ChangeNotifierMixin {
-  static const _VALUE = const Symbol('value');
+class _Binding extends ChangeNotifierBase {
 
   final Scope _scope;
   final ExpressionObserver _expr;
@@ -125,7 +124,7 @@ class _Binding extends Object with ChangeNotifierMixin {
     } else {
       _value = (_converter == null) ? v : _converter(v);
     }
-    notifyChange(new PropertyChangeRecord(_VALUE));
+    notifyChange(new PropertyChangeRecord(#value));
   }
 
   get value => _value;
@@ -133,17 +132,17 @@ class _Binding extends Object with ChangeNotifierMixin {
   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;
   }
 }