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

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 7b1715456fb692d4587ca85eabda81f4f3046005..66ae044cf26a20dd846535efce55619a9ad5d80b 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -43724,9 +43724,18 @@ 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)) {
+        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.
+          _removeNode(child, node);
+          walk(node, parent);
+          return;
+        }
         walk(child, node);
         child = nextChild;
       }