Fix bug in polymer expressions where input-value bindings are incorrectly

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..3cee82067c542d0d11497846e217746156900e14 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -91,8 +91,6 @@ class PolymerExpressions extends BindingDelegate {
 }
 
 class _Binding extends Object with ChangeNotifierMixin {
-  static const _VALUE = const Symbol('value');
-
   final Scope _scope;
   final ExpressionObserver _expr;
   final _converter;
@@ -113,6 +111,7 @@ class _Binding extends Object with ChangeNotifierMixin {
   }
 
   _setValue(v) {
+    var oldValue = _value;
     if (v is Comprehension) {
       // convert the Comprehension into a list of scopes with the loop
       // variable added to the scope
@@ -125,7 +124,7 @@ class _Binding extends Object with ChangeNotifierMixin {
     } else {
       _value = (_converter == null) ? v : _converter(v);
     }
-    notifyChange(new PropertyChangeRecord(_VALUE));
+    notifyPropertyChange(#value, oldValue, _value);
   }
 
   get value => _value;
@@ -133,17 +132,8 @@ class _Binding extends Object with ChangeNotifierMixin {
   set value(v) {
     try {
       assign(_expr, v, _scope);
-      notifyChange(new PropertyChangeRecord(_VALUE));
     } on EvalException catch (e) {
       // silently swallow binding errors
     }
   }
-
-  getValueWorkaround(key) {
-    if (key == _VALUE) return value;
-  }
-
-  setValueWorkaround(key, v) {
-    if (key == _VALUE) value = v;
-  }
 }