Add two way data binding.

sky/framework/sky-element/sky-element.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.
 -->
 <import src="sky-binder.sky" as="binder" />
 <script>
 var attributeConverters = {
   boolean: function(value) {
     if (typeof value == 'string')
@@ skipping 142 matching lines @@
   attributeChanged(attrName, oldValue, newValue) {
     // override
   }
 
   shadowRootReady() {
     // override
   }
 
   createdCallback() {
     this.isAttached = false;
+    this.propertyBindings = null;
+    this.dirtyPropertyBindings = null;
     this.created();
 
     Object.preventExtensions(this);
 
     // Invoke attributeChanged callback when element is first created too.
     var attributes = this.getAttributes();
     for (var i = 0; i < attributes.length; ++i) {
       var attribute = attributes[i];
       this.attributeChangedCallback(attribute.name, null, attribute.value);
     }
@@ skipping 28 matching lines @@
     this.attributeChanged(name, oldValue, newValue);
     var registration = registrations.get(this.localName);
     var converter = registration.attributes.get(name);
     if (converter) {
       this.notifyPropertyChanged(name, converter(oldValue),
           converter(newValue));
     }
   }
 
   notifyPropertyChanged(name, oldValue, newValue) {
+    if (oldValue == newValue)
+      return;
     var notifier = Object.getNotifier(this);
     notifier.notify({
       type: 'update',
       name: name,
       oldValue: oldValue,
     });
     var handler = this[name + 'Changed'];
     if (typeof handler == 'function')
       handler.call(this, oldValue, newValue);
+    this.schedulePropertyBindingUpdate(name);
+  }
+
+  addPropertyBinding(name, binding) {
+    if (!this.propertyBindings)
+      this.propertyBindings = new Map();
+    this.propertyBindings.set(name, binding);
+  }
+
+  getPropertyBinding(name) {
+    if (!this.propertyBindings)
+      return null;
+    return this.propertyBindings.get(name);
+  }
+
+  schedulePropertyBindingUpdate(name) {
+    if (!this.dirtyPropertyBindings) {
+      this.dirtyPropertyBindings = new Set();
+      Promise.resolve().then(this.updatePropertyBindings.bind(this));
+    }
+    this.dirtyPropertyBindings.add(name);
+  }
+
+  updatePropertyBindings() {
+    for (var name of this.dirtyPropertyBindings) {
+      var binding = this.getPropertyBinding(name);
+      if (binding) {
+        binding.setValue(this[name]);
+        binding.discardChanges();
+      }
+    }
+    this.dirtyPropertyBindings = null;
   }
 };
 
 module.exports = SkyElement;
 </script>