Support optional, non-defaulted constructor arguments.

Source/bindings/tests/idls/TestInterfaceNamedConstructor.idl

 /*
  * Copyright (C) 2011 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:
  *
  * 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 15 matching lines @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
     ActiveDOMObject,
     NamedConstructor=Audio(
         DOMString stringArg,
         [Default=Undefined] optional boolean defaultUndefinedOptionalBooleanArg,
         [Default=Undefined] optional long defaultUndefinedOptionalLongArg,
         [Default=Undefined] optional DOMString defaultUndefinedOptionalStringArg,
-        [Default=NullString] optional DOMString defaultNullStringOptionalstringArg),
+        [Default=NullString] optional DOMString defaultNullStringOptionalstringArg,
+        optional DOMString optionalStringArg),
     RaisesException=Constructor,
 ] interface TestInterfaceNamedConstructor {
     // An attribute of type {interface_name}ConstructorConstructor is generated
     // in *Constructors.idl file, which is a partial interface for the global
     // object, e.g. Window. We put this here to avoid another test file.
     attribute TestNamedConstructorConstructor testNamedConstructorConstructorAttribute;
 };