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

tools/dom/src/Validators.dart

Index: tools/dom/src/Validators.dart
diff --git a/tools/dom/src/Validators.dart b/tools/dom/src/Validators.dart
index 4dbe32c5942269275d38bf590e5d6a5cf60e1c0f..249ddeeb1b06901ba85fe33d83a3a4a84eaf0760 100644
--- a/tools/dom/src/Validators.dart
+++ b/tools/dom/src/Validators.dart
@@ -167,9 +167,18 @@ class _ValidatingTreeSanitizer implements NodeTreeSanitizer {
       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;
       }