Fix bug in polymer expressions where input-value bindings are incorrectly

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 106 matching lines @@
       _value = (_converter == null) ? v : _converter(v);
     }
     notifyChange(new PropertyChangeRecord(_VALUE));
   }
 
   get value => _value;
 
   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
     }
   }
 
   getValueWorkaround(key) {
     if (key == _VALUE) return value;
   }
 
   setValueWorkaround(key, v) {
     if (key == _VALUE) value = v;
   }
 
 }