Add method to html_dartium utils to check for NSM errors

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index eae77bd0b6ca15265a7d1b063b09b68897f68c2f..ad04f3fe91adde83434f595fc7cefafee7045e8b 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -189,6 +189,8 @@ class _Utils {
 
   static String addTrailingDot(String str) => '${str}.';
 
+  static bool isNoSuchMethodError(obj) => obj is NoSuchMethodError;
+
   // TODO(jacobr): we need a failsafe way to determine that a Node is really a
   // DOM node rather than just a class that extends Node.
   static bool isNode(obj) => obj is Node;