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 e7148ad98be8ebce9e7b83f23d109da3929588a1..e5dbb7d498d119c20d641b5788a24660e9438de3 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -124,7 +124,6 @@ class _Binding extends Object with ChangeNotifierMixin {
   set value(v) {
     try {
       assign(_expr, v, _scope);
-      notifyChange(new PropertyChangeRecord(_VALUE));

[Siggi Cherem (dart-lang), 2013/10/09 03:25:54]

to clarify:
- a change in the input element will update the binding here
- assign will update _expr
- expr.onUpdate will trigger calling _setValue above
- which will notify about the change in line 119 for a second time.

[justinfagnani, 2013/10/09 19:00:59]

I think I tried removing this and ran into other issues. Can you try the large
example and see if other uses of the value bound here update? 

[Jennifer Messerly, 2013/10/09 19:04:37]

probably the right fix is:

try {
  var oldValue = _value;
  assign(...);
  notifyPropertyChange(#value, oldValue, _value);
} ...

ditto for the other notifyChange above :)

     } on EvalException catch (e) {
       // silently swallow binding errors
     }