Prevent expandos on all SkyElements.

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 templates = new Map();
 
 var attributeConverters = {
@@ skipping 83 matching lines @@
   }
 
   shadowRootReady() {
     // override
   }
 
   createdCallback() {
     this.isAttached = false;
     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);
     }
   }
 
   attachedCallback() {
     if (!this.shadowRoot) {
@@ skipping 29 matching lines @@
       oldValue: oldValue,
     });
     var handler = this[name + 'Changed'];
     if (typeof handler == 'function')
       handler.call(this, oldValue, newValue);
   }
 };
 
 module.exports = SkyElement;
 </script>