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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 80f607fc2eeccb90f237fb184fb6c5e36122e072..880109cae3c4336ebfbb3776f623cd31954081a2 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -40404,6 +40404,12 @@ 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;
     }