Make sanitizer tests handle IE11, where tagName can't be clobbered

tests/html/node_validator_important_if_you_suppress_make_the_bug_critical_test.dart

Index: tests/html/node_validator_important_if_you_suppress_make_the_bug_critical_test.dart
diff --git a/tests/html/node_validator_important_if_you_suppress_make_the_bug_critical_test.dart b/tests/html/node_validator_important_if_you_suppress_make_the_bug_critical_test.dart
index 34e29bb54ad15732d2ce8c6f2e86c6ebb4d3ff3e..a2a23c6823e390c738df00ea26a971f5c6e791f9 100644
--- a/tests/html/node_validator_important_if_you_suppress_make_the_bug_critical_test.dart
+++ b/tests/html/node_validator_important_if_you_suppress_make_the_bug_critical_test.dart
@@ -529,14 +529,33 @@ main() {
     "<input id='bad' onmouseover='alert(1)'>",
     "");
 
-    testHtml('tagName makes containing form invalid',
-    validator,
-    "<form onmouseover='alert(2)'><input name='tagName'>",
-    "");
+    test('tagName makes containing form invalid', () {
+      var fragment = document.body.createFragment(
+          "<form onmouseover='alert(2)'><input name='tagName'>",
+          validator: validator);
+      var form = fragment.lastChild;
+      // If the tagName was clobbered, the sanitizer should have removed
+      // the whole thing and form is null.
+      // If the tagName was not clobbered, then there will be content,
+      // but the tagName should be the normal value. IE11 has started
+      // doing this.
+      if (form != null) {
+        expect(form.tagName, 'FORM');
+      }
+    });
 
-    testHtml('tagName without mouseover',
-    validator,
-    "<form><input name='tagName'>",
-    "");
+    test('tagName without mouseover', () {
+      var fragment = document.body.createFragment(
+          "<form><input name='tagName'>",
+          validator: validator);
+      var form = fragment.lastChild;
+      // If the tagName was clobbered, the sanitizer should have removed
+      // the whole thing and form is null.
+      // If the tagName was not clobbered, then there will be content,
+      // but the tagName should be the normal value.
+      if (form != null) {
+        expect(form.tagName, 'FORM');
+      }
+    });
   });
 }