Support union return type for anonymous named/indexed getter

Source/bindings/tests/idls/TestInterface.idl

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary formstrArg, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 18 matching lines @@
 // This IDL file is for testing the bindings code generator and for tracking
 // changes in its ouput.
 [
     ActiveDOMObject,
     CustomNamedSetter,
     Conditional=Condition1|Condition2,
     CallWith=ScriptExecutionContext,
     Constructor(DOMString str1, [Default=Undefined] optional DOMString str2),
     RaisesException
 ] interface TestInterface {
+    [ImplementedAs=getItem] getter (Node or NodeList)(DOMString name);
 };