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

tools/dom/src/NodeValidatorBuilder.dart

Index: tools/dom/src/NodeValidatorBuilder.dart
diff --git a/tools/dom/src/NodeValidatorBuilder.dart b/tools/dom/src/NodeValidatorBuilder.dart
index 41cab67cf9b140830fe4f3f189ecf9f6af5b1e85..7789c1b017df9cce1f314b7472a1b0140ca14b0d 100644
--- a/tools/dom/src/NodeValidatorBuilder.dart
+++ b/tools/dom/src/NodeValidatorBuilder.dart
@@ -453,6 +453,12 @@ 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;
     }