big update to observe, template_binding, polymer

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';
 
 @MirrorsUsed(metaTargets: const [Reflectable, ObservableProperty],
@@ skipping 231 matching lines @@
   ExpressionObserver _parent;
 
   StreamSubscription _subscription;
   Object _value;
 
   StreamController _controller = new StreamController.broadcast();
   Stream get onUpdate => _controller.stream;
 
   ExpressionObserver(this._expr);
 
+  Expression get expression => _expr;
+
   Object get currentValue => _value;
 
   update(Scope scope) => _updateSelf(scope);
 
   _updateSelf(Scope scope) {}
 
   _invalidate(Scope scope) {
     _observe(scope);
     if (_parent != null) {
       _parent._invalidate(scope);
@@ skipping 481 matching lines @@
    * This does not work for calls that need to pass more than one argument.
    */
   call(arg0) => mirror.invoke(symbol, [arg0], null).reflectee;
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }