Fix two-way binding in polymer expressions.

pkg/polymer_expressions/lib/eval.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.
 
 library polymer_expressions.eval;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:observe/observe.dart';
@@ skipping 62 matching lines @@
 }
 
 /**
  * Assign [value] to the variable or field referenced by [expr] in the context
  * of [scope].
  *
  * [expr] must be an /assignable/ expression, it must not contain
  * operators or function invocations, and any index operations must use a
  * literal index.
  */
-void assign(Expression expr, Object value, Scope scope) {
+Object assign(Expression expr, Object value, Scope scope) {
 
   notAssignable() =>
       throw new EvalException("Expression is not assignable: $expr");
 
   Expression expression;
   var property;
   bool isIndex = false;
   var filters = <Expression>[]; // reversed order for assignment
 
   while (expr is BinaryOperator) {
@@ skipping 39 matching lines @@
     value = filter.reverse(value);
   }
   // make the assignment
   var o = eval(expression, scope);
   if (o == null) throw new EvalException("Can't assign to null: $expression");
   if (isIndex) {
     o[property] = value;
   } else {
     smoke.write(o, smoke.nameToSymbol(property), value);
   }
+  return value;
 }
 
 
 /**
  * A scope in polymer expressions that can map names to objects. Scopes contain
  * a set of named variables and a unique model object. The scope structure
  * is then used to lookup names using the `[]` operator. The lookup first
  * searches for the name in local variables, then in global variables,
  * and then finally looks up the name as a property in the model.
  */
@@ skipping 591 matching lines @@
 
   Comprehension(this.identifier, Iterable iterable)
       : iterable = (iterable != null) ? iterable : const [];
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }