Fix two-way binding in polymer expressions.

pkg/polymer_expressions/lib/eval.dart

Index: pkg/polymer_expressions/lib/eval.dart
diff --git a/pkg/polymer_expressions/lib/eval.dart b/pkg/polymer_expressions/lib/eval.dart
index 2332cf908f6f276aea268197cba996be800abaa3..c33a262ad510831dbd9f5b56dc0c55a8bc8f18a4 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -80,7 +80,7 @@ Object update(ExpressionObserver expr, Scope scope) {
  * 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");
@@ -140,6 +140,7 @@ void assign(Expression expr, Object value, Scope scope) {
   } else {
     smoke.write(o, smoke.nameToSymbol(property), value);
   }
+  return value;
 }