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;
+      while (!identical(child, null)) {

[sra1, 2016/03/25 17:20:12]

!= null is fine - null as an explicit argument is always handled specially. If
your are still nervous:

null != child

the receiver is null so it has known behaviour.

[Alan Knight, 2016/03/25 18:10:46]

OK, just being extremely paranoid. Made it 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 and restart the traversal without it.

[sra1, 2016/03/25 17:20:12]

Say why a restart is necessary instead of just removing the node.

[Alan Knight, 2016/03/25 18:10:46]

Added comment. Rewrote it to not be recursive.

+          _removeNode(child, node);
+          walk(node, parent);
+          return;
+        }
         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
@@ skipping 104 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);
     }
   }
 }