console.error when using unknown attributes in templates.

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" />
 
 <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();
 
+// TODO(esprehn): It would be nice if these were exposed by the platform so
+// the framework didn't need to hard code a list.
+var defaultAttributesNames = new Set([
+  'accesskey',
+  'alt',
+  'as',
+  'async',
+  'class',
+  'contenteditable',
+  'crossorigin',
+  'dir',
+  'height',
+  'href',
+  'id',
+  'is',
+  'lang',
+  'media',
+  'name',
+  'rel',
+  'select',
+  'sizes',
+  'spellcheck',
+  'src',
+  'srcset',
+  'style',
+  'tabindex',
+  'title',
+  'type',
+  'width',
+]);
+var elementAttributeNames = new Map();
+
+function registerElement(name, options) {
+  elementAttributeNames.set(name, new Set(options.attributeNames));
+}
+
+registerElement('template', {
+  attributeNames: ['if', 'repeat'],
+});
+
 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 108 matching lines @@
       result = new PropertyDirective(property);
     result.expressions.push(new BindingExpression(prefix, path));
   }
 
   if (result && offset < value.length)
     result.suffix = value.substring(offset);
 
   return result;
 }
 
+function checkAttribute(name, allowedAttributeNames) {
+  if (name.startsWith('data-'))
+    return true;
+  if (defaultAttributesNames.has(name))
+    return true;
+  if (allowedAttributeNames && allowedAttributeNames.has(name))
+    return true;
+  return false;
+}
+
 function parseAttributeDirectives(element, directives) {
   var attributes = element.getAttributes();
 
+  var allowedAttributeNames = elementAttributeNames.get(element.tagName);
+
   for (var i = 0; i < attributes.length; i++) {
     var attr = attributes[i];
     var name = attr.name;
     var value = attr.value;
 
     if (name.startsWith('on-')) {
       directives.eventHandlers.push(name.substring(3));
       continue;
     }
 
+    if (!checkAttribute(name, allowedAttributeNames)) {
+      console.error('Element "'+ element.tagName +
+          '" has unknown attribute "' + name + '".');
+    }
+
     var property = parsePropertyDirective(value, name);
     if (!property)
       continue;
 
     directives.properties.push(property);
   }
 }
 
 function eventHandlerCallback(event) {
   var element = event.currentTarget;
@@ skipping 280 matching lines @@
     if (this.valueObserver)
       this.valueObserver.close();
 
     iterators.delete(this.template);
     this.closed = true;
   }
 }
 
 module.exports = {
   createInstance: createInstance,
+  registerElement: registerElement,
 };
 </script>