Sanitization should reject elements that we can't examine (e.g. embed/object on FF)

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 4a5fe71550fd2b61ecb64cbb17e4fa10b4fbc4a1..34e29bb54ad15732d2ce8c6f2e86c6ebb4d3ff3e 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
@@ -154,7 +154,12 @@ main() {
       expect(fragment.childNodes.length, 1);
       expect(fragment.childNodes[0].id, "bad");
       expect(fragment.childNodes[0].childNodes.length, 0);
-    });    
+    });
+
+    testHtml("sanitizes embed",
+      validator,
+      "<div><embed src='' type='application/x-shockwave-flash'></embed></div>",
+      "<div></div>");
   });
 
   group('URI_sanitization', () {