Fix two-way binding in polymer expressions.

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 105 matching lines @@
     try {
       return _convertValue(eval(expr, scope), scope, converter);
     } catch (e, s) {
       new Completer().completeError(
           "Error evaluating expression '$expr': $e", s);
     }
     return null;
   }
 
   _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) {
     if (v is Comprehension) {
       // convert the Comprehension into a list of scopes with the loop
       // variable added to the scope
       return v.iterable.map((i) => scope.childScope(v.identifier, i))
           .toList(growable: false);
     } else {
       return converter == null ? v : converter(v);
     }
   }
 
   get value {
     if (_callback != null) return _value;
     return _oneTime(_expr, _scope, _converter);
   }
 
   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);
     }
   }
 
   open(callback(value)) {
     if (_callback != null) throw new StateError('already open');
 
     _callback = callback;
@@ skipping 15 matching lines @@
 
   void close() {
     if (_callback == null) return;
 
     _sub.cancel();
     _sub = null;
     _expr = (_expr as ExpressionObserver).expression;
     _callback = null;
   }
 }