dart2js_html: Fix for issue 29538 - some returned lists may be null

tools/dom/scripts/systemhtml.py

 #!/usr/bin/python
 # 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.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 import logging
@@ skipping 1224 matching lines @@
     anns = self._metadata.GetDart2JSMetadata(
         idl_type, self._library_name, self._interface, idl_member_name)
 
     if not self._metadata.AnyConversionAnnotations(
         idl_type, self._interface.id, idl_member_name):
       return_type = self.SecureOutputType(idl_type)
       native_type = self._NarrowToImplementationType(idl_type)
 
       if native_type != return_type:
         anns = anns + [
-          "@Returns('%s')" % native_type,
+          "@Returns('%s|Null')" % native_type,
           "@Creates('%s')" % native_type,
         ]
     if dart_type == 'dynamic' or dart_type == 'Object':
       def js_type_annotation(ann):
         return re.search('^@.*Returns', ann) or re.search('^@.*Creates', ann)
       if not filter(js_type_annotation, anns):
         _logger.warn('Member with wildcard native type: %s.%s' %
             (self._interface.id, idl_member_name))
 
     return self._metadata.FormatMetadata(anns, indent);
@@ skipping 123 matching lines @@
 
   def AddFile(self, basename, library_name, path):
     self._libraries[library_name].AddFile(path)
 
   def AddTypeEntry(self, library_name, idl_name, dart_name):
     self._libraries[library_name].AddTypeEntry(idl_name, dart_name)
 
   def Emit(self, emitter, auxiliary_dir):
     for lib in self._libraries.values():
       lib.Emit(emitter, auxiliary_dir)