Add a basic custom painting facility to Sky

sky/engine/bindings/scripts/dart_callback_interface.py

 # Copyright (C) 2013 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
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 106 matching lines @@
             'handle': '%sHandle' % argument.name,
             'cpp_value_to_dart_value': argument.idl_type.cpp_value_to_dart_value(argument.name,
                                                                                  creation_context=creation_context),
         }
 
     argument_declarations = [
             '%s %s' % (argument.idl_type.callback_cpp_type, argument.name)
             for argument in arguments]
     if call_with_this_handle:
         argument_declarations.insert(0, 'ScriptValue thisValue')
+
+    dart_argument_declarations = [
+            '%s %s' % (dart_types.idl_type_to_dart_type(argument.idl_type), argument.name)
+            for argument in arguments]
     return  {
         'argument_declarations': argument_declarations,
+        'dart_argument_declarations': dart_argument_declarations,
         'arguments': [generate_argument(argument) for argument in arguments],
     }