[mojo-bindings] Support reuse of native enum classes

mojo/public/tools/bindings/pylib/mojom/parse/ast.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Node classes for the AST for a Mojo IDL file."""
 
 # Note: For convenience of testing, you probably want to define __eq__() methods
 # for all node types; it's okay to be slightly lax (e.g., not compare filename
 # and lineno). You may also define __repr__() to help with analyzing test
 # failures, especially for more complex types.
@@ skipping 120 matching lines @@
     return super(Const, self).__eq__(other) and \
            self.typename == other.typename and \
            self.value == other.value
 
 
 class Enum(Definition):
   """Represents an enum definition."""
 
   def __init__(self, name, attribute_list, enum_value_list, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
-    assert isinstance(enum_value_list, EnumValueList)
+    assert isinstance(enum_value_list, EnumValueList) or enum_value_list is None
     super(Enum, self).__init__(name, **kwargs)
     self.attribute_list = attribute_list
     self.enum_value_list = enum_value_list
 
   def __eq__(self, other):
     return super(Enum, self).__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.enum_value_list == other.enum_value_list
 
 
@@ skipping 249 matching lines @@
   def __eq__(self, other):
     return super(UnionField, self).__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.ordinal == other.ordinal and \
            self.typename == other.typename
 
 
 class UnionBody(NodeListBase):
 
   _list_item_type = UnionField