Support custom to native conversions.

lib/dom/templates/dom/native/cpp_header.template

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 
 #ifndef Dart$(INTERFACE)_h
 #define Dart$(INTERFACE)_h
 
 #include "DartDOMWrapper.h"
 $WEBCORE_INCLUDES
 #include <dart_api.h>
 
 namespace WebCore {
 
 struct Dart$INTERFACE {
     static const char* const dartImplementationClassName;
     typedef $WEBCORE_CLASS_NAME NativeType;
     typedef $NATIVE_TRAITS_TYPE NativeTraits;
 
-    static PassRefPtr<NativeType> toNative(Dart_Handle handle, Dart_Handle& exception)
-    {
-        return DartDOMWrapper::unwrapDartWrapper<Dart$INTERFACE>(handle, exception);
-    }
-
+$TO_NATIVE
     static Dart_NativeFunction resolver(Dart_Handle name, int argumentCount);
 };
 
-$TO_DART_VALUE
+$TO_DART
 inline Dart_Handle toDartValue(PassRefPtr< $WEBCORE_CLASS_NAME > value)
 {
     return toDartValue(value.get());
 }
 
 namespace Dart$(INTERFACE)Internal {
 $DECLARATIONS
 }
 
 }
 
 #endif // Dart$(INTERFACE)_h