Interpret Dart null as both null and undefined.

Source/bindings/dart/DartScriptValue.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 85 matching lines @@
 
     bool isNull() const
     {
         ASSERT(!isEmpty());
         return Dart_IsNull(m_value);
     }
 
     bool isUndefined() const
     {
         ASSERT(!isEmpty());
-        return false;
+        return Dart_IsNull(m_value);
     }
 
     bool isObject() const
     {
         ASSERT(!isEmpty());
         return true;
     }
 
     bool isEmpty() const
     {
@@ skipping 22 matching lines @@
     explicit DartScriptValue(DartScriptState* scriptState, Dart_Handle value)
         : m_scriptState(scriptState)
         , m_value(value)
     {
     }
 };
 
 } // namespace WebCore
 
 #endif // DartScriptValue_h