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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index bbb5d14822c8b830ede0351375230e2b5012f76c..e5d710a6e0ff0d011676dd52eecd5da094388a29 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -50251,10 +50251,21 @@ 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;
-        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;
       }
     }