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

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 14bfb3eef1fd7b9201cb072440eef82c9c380e5a..061ccc495553295d94f9c4b94663d4b945a23101 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -38181,6 +38181,12 @@ class _SvgNodeValidator implements NodeValidator {
     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;
     }