Adding warning message when HTML contents get sanitized away.

tools/dom/src/Validators.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.dom.html;
 
 
 /**
  * Interface used to validate that only accepted elements and attributes are
  * allowed while parsing HTML strings into DOM nodes.
@@ skipping 147 matching lines @@
     }
     walk(node);
   }
 
   void sanitizeNode(Node node) {
     switch (node.nodeType) {
       case Node.ELEMENT_NODE:
         Element element = node;
         var attrs = element.attributes;
         if (!validator.allowsElement(element)) {
+          window.console.warn(
+              'Removing disallowed element <${element.tagName}>');
           element.remove();
           break;
         }
 
         var isAttr = attrs['is'];
         if (isAttr != null) {
           if (!validator.allowsAttribute(element, 'is', isAttr)) {
+            window.console.warn('Removing disallowed type extension '
+                '<${element.tagName} is="$isAttr">');
             element.remove();
             break;
           }
         }
 
         // TODO(blois): Need to be able to get all attributes, irrespective of
         // XMLNS.
         var keys = attrs.keys.toList();
         for (var i = attrs.length - 1; i >= 0; --i) {
           var name = keys[i];
           if (!validator.allowsAttribute(element, name.toLowerCase(),
               attrs[name])) {
+            window.console.warn('Removing disallowed attribute '
+                '<${element.tagName} $name="${attrs[name]}">');
             attrs.remove(name);
           }
         }
 
         if (element is TemplateElement) {
           TemplateElement template = element;
           sanitizeTree(template.content);
         }
         break;
       case Node.COMMENT_NODE:
       case Node.DOCUMENT_FRAGMENT_NODE:
       case Node.TEXT_NODE:
       case Node.CDATA_SECTION_NODE:
         break;
       default:
         node.remove();
     }
   }
 }