Add two way data binding.

sky/framework/sky-element/observe.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 
 <script>
 function detectEval() {
   // Don't test for eval if we're running in a Chrome App environment.
   // We check for APIs set that only exist in a Chrome App context.
@@ skipping 583 matching lines @@
     if (this.state_ != UNOPENED)
       throw Error('Observer has already been opened.');
 
     this.callback_ = callback;
     this.target_ = target;
     this.connect_();
     this.state_ = OPENED;
     return this.value_;
   },
 
+  setValue: function(newValue) {
+  },
+
   close: function() {
     if (this.state_ != OPENED)
       return;
 
     this.disconnect_();
     this.value_ = undefined;
     this.callback_ = undefined;
     this.target_ = undefined;
     this.state_ = CLOSED;
   },
@@ skipping 266 matching lines @@
 
     // TODO(rafaelw): Having observed_ as the third callback arg here is
     // pretty lame API. Fix.
     this.report_([this.value_, oldValues, this.observed_]);
     return true;
   }
 });
 
 function identFn(value) { return value; }
 
-function ObserverTransform(observable, getValueFn, setValueFn,
-                           dontPassThroughSet) {
+function ObserverTransform(observable, getValueFn) {
   this.callback_ = undefined;
   this.target_ = undefined;
   this.value_ = undefined;
   this.observable_ = observable;
   this.getValueFn_ = getValueFn || identFn;
-  this.setValueFn_ = setValueFn || identFn;
-  // TODO(rafaelw): This is a temporary hack. PolymerExpressions needs this
-  // at the moment because of a bug in it's dependency tracking.
-  this.dontPassThroughSet_ = dontPassThroughSet;
 }
 
 ObserverTransform.prototype = {
   open: function(callback, target) {
     this.callback_ = callback;
     this.target_ = target;
     this.value_ =
         this.getValueFn_(this.observable_.open(this.observedCallback_, this));
     return this.value_;
   },
 
   observedCallback_: function(value) {
     value = this.getValueFn_(value);
     if (areSameValue(value, this.value_))
       return;
     var oldValue = this.value_;
     this.value_ = value;
     this.callback_.call(this.target_, this.value_, oldValue);
   },
 
+  setValue: function(oldValue) {
+  },
+
   discardChanges: function() {
     this.value_ = this.getValueFn_(this.observable_.discardChanges());
     return this.value_;
   },
 
   deliver: function() {
     return this.observable_.deliver();
   },
 
-  setValue: function(value) {
-    value = this.setValueFn_(value);
-    if (!this.dontPassThroughSet_ && this.observable_.setValue)
-      return this.observable_.setValue(value);
-  },
-
   close: function() {
     if (this.observable_)
       this.observable_.close();
     this.callback_ = undefined;
     this.target_ = undefined;
     this.observable_ = undefined;
     this.value_ = undefined;
     this.getValueFn_ = undefined;
-    this.setValueFn_ = undefined;
   }
 }
 
 function newSplice(index, removed, addedCount) {
   return {
     index: index,
     removed: removed,
     addedCount: addedCount
   };
 }
@@ skipping 403 matching lines @@
   runEOM_: runEOM,
   observerSentinel_: observerSentinel,
   PathObserver: PathObserver,
   ArrayObserver: ArrayObserver,
   ArraySplice: ArraySplice,
   CompoundObserver: CompoundObserver,
   Path: Path,
   ObserverTransform: ObserverTransform
 }
 </script>