Make SVGValidator reject foreignobject tags, not just their contents. Fixes Firefox inconsistency

tools/dom/src/NodeValidatorBuilder.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.dom.html;
 
 
 /**
  * Class which helps construct standard node validation policies.
  *
@@ skipping 435 matching lines @@
     return false;
   }
 }
 
 
 class _SvgNodeValidator implements NodeValidator {
   bool allowsElement(Element element) {
     if (element is svg.ScriptElement) {
       return false;
     }
+    // Firefox 37 has issues with creating foreign elements inside a
+    // foreignobject tag as SvgElement. We don't want foreignobject contents
+    // anyway, so just remove the whole tree outright.
+    if (element is svg.ForeignObjectElement) {
+      return false;
+    }
     if (element is svg.SvgElement) {
       return true;
     }
     return false;
   }
 
   bool allowsAttribute(Element element, String attributeName, String value) {
     if (attributeName == 'is' || attributeName.startsWith('on')) {
       return false;
     }
     return allowsElement(element);
   }
 }