Fix two-way binding in polymer expressions.

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 f26c2f3e7e58a91655ac7981b07036be170902de..ac939d8ecb914ca496a07bf9012e5daa8cd48b1e 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -123,8 +123,9 @@ class _Binding extends Bindable {
   }
 
   _setValue(v) {
+    var oldValue = _value;
     _value = _convertValue(v, _scope, _converter);
-    if (_callback != null) _callback(_value);
+    if (_callback != null && oldValue != _value) _callback(_value);
   }
 
   static _convertValue(v, scope, converter) {
@@ -145,7 +146,8 @@ class _Binding extends Bindable {
 
   set value(v) {
     try {
-      assign(_expr, v, _scope);
+      var newValue = assign(_expr, v, _scope);
+      _value = _convertValue(newValue, _scope, _converter);
     } catch (e, s) {
       new Completer().completeError(
           "Error evaluating expression '$_expr': $e", s);