Be more strict about attributes in sky-binder.

sky/framework/sky-element/sky-binder.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="observe.sky" as="observe" />
 <import src="element-registry.sky" as="registry" />
 
 <script>
 var stagingDocument = new Document();
 
 class TemplateInstance {
   constructor() {
     this.bindings = [];
     this.terminator = null;
     this.fragment = stagingDocument.createDocumentFragment();
     Object.preventExtensions(this);
   }
   close() {
     var bindings = this.bindings;
     for (var i = 0; i < bindings.length; i++) {
       bindings[i].close();
     }
   }
 }
 
 var emptyInstance = new TemplateInstance();
 var directiveCache = new WeakMap();
 
-(function() {
-  var templateRegistration = registry.registerElement('template');
-  templateRegistration.defineAttribute('if', 'string');
-  templateRegistration.defineAttribute('repeat', 'string');
-})();
-
 function createInstance(template, model) {
   var content = template.content;
   if (!content.firstChild)
     return emptyInstance;
 
   var directives = directiveCache.get(content);
   if (!directives) {
     directives = new NodeDirectives(content);
     directiveCache.set(content, directives);
   }
@@ skipping 431 matching lines @@
 
     iterators.delete(this.template);
     this.closed = true;
   }
 }
 
 module.exports = {
   createInstance: createInstance,
 };
 </script>