Conversion of Promise is trivial

Source/bindings/scripts/v8_types.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 465 matching lines @@
 
 IdlType.v8_conversion_needs_exception_state = property(v8_conversion_needs_exception_state)
 IdlArrayOrSequenceType.v8_conversion_needs_exception_state = True
 
 
 TRIVIAL_CONVERSIONS = frozenset([
     'any',
     'boolean',
     'Dictionary',
     'NodeFilter',
-    'XPathNSResolver'
+    'XPathNSResolver',
+    'Promise'
 ])
 
 
 def v8_conversion_is_trivial(idl_type):
     # The conversion is a simple expression that returns the converted value and
     # cannot raise an exception.
     return (idl_type.base_type in TRIVIAL_CONVERSIONS or
             idl_type.is_wrapper_type)
 
 IdlType.v8_conversion_is_trivial = property(v8_conversion_is_trivial)
@@ skipping 346 matching lines @@
 
 
 def is_explicit_nullable(idl_type):
     # Nullable type that isn't implicit nullable (see above.) For such types,
     # we use Nullable<T> or similar explicit ways to represent a null value.
     return idl_type.is_nullable and not idl_type.is_implicit_nullable
 
 IdlTypeBase.is_implicit_nullable = property(is_implicit_nullable)
 IdlUnionType.is_implicit_nullable = False
 IdlTypeBase.is_explicit_nullable = property(is_explicit_nullable)