Add ImplementedBy attribute for operations merged from [Supplemental=XXX] interfaces.

client/dom/scripts/databasebuilder.py

 #!/usr/bin/python
 # Copyright (c) 2011, 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.
 
 import copy
 import database
 import idlparser
 import logging
 import os
@@ skipping 342 matching lines @@
     source = import_options.source
     if (source and source not in old_interface.annotations and
         source in new_interface.annotations and
         not new_interface.is_supplemental):
       old_interface.annotations[source] = new_interface.annotations[source]
       changed = True
 
     def merge_list(what):
       old_list = old_interface.__dict__[what]
       new_list = new_interface.__dict__[what]
+
+      if what != 'parents' and old_interface.id != new_interface.id:
+        for node in new_list:
+          node.ext_attrs['ImplementedBy'] = new_interface.id
+
       changed = self._merge_nodes(old_list, new_list, import_options)
 
       # Delete list items with zero remaining annotations.
       if changed and import_options.obsolete_old_declarations:
 
         def has_annotations(idl_node):
           return len(idl_node.annotations)
 
         old_interface.__dict__[what] = filter(has_annotations, old_list)
 
@@ skipping 222 matching lines @@
             annotation = idl_node.annotations[source]
             for name, value in annotation.items():
               if (name in top_level_annotation
                   and value == top_level_annotation[name]):
                 del annotation[name]
 
         map(normalize, interface.parents)
         map(normalize, interface.constants)
         map(normalize, interface.attributes)
         map(normalize, interface.operations)