Check for the case where previousNode fails in sanitization. Can happen with object tags

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 149 matching lines @@
  */
 class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
   NodeValidator validator;
   _ValidatingTreeSanitizer(this.validator) {}
 
   void sanitizeTree(Node node) {
     void walk(Node node, Node parent) {
       sanitizeNode(node, parent);
 
       var child = node.lastChild;
-      while (child != null) {
-        // Child may be removed during the walk.
-        var nextChild = child.previousNode;
-        walk(child, node);
+      while (null != child) {
+        var nextChild;
+        try {
+          // Child may be removed during the walk, and we may not
+          // even be able to get its previousNode.
+          nextChild = child.previousNode;
+        } catch (e) {
+          // Child appears bad, remove it. We want to check the rest of the
+          // children of node and, but we have no way of getting to the next
+          // child, so start again from the last child.
+          _removeNode(child, node);
+          child = null;
+          nextChild = node.lastChild;
+        }
+        if (child != null) walk(child, node);
         child = nextChild;
       }
     }
     walk(node, null);
   }
 
   /// Aggressively try to remove node.
   void _removeNode(Node node, Node parent) {
     // If we have the parent, it's presumably already passed more sanitization
     // or is the fragment, so ask it to remove the child. And if that fails
@@ skipping 103 matching lines @@
       case Node.COMMENT_NODE:
       case Node.DOCUMENT_FRAGMENT_NODE:
       case Node.TEXT_NODE:
       case Node.CDATA_SECTION_NODE:
         break;
       default:
         _removeNode(node, parent);
     }
   }
 }