Generate contains() method for list mixin only if not already defined.

lib/html/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 os
@@ skipping 680 matching lines @@
             '\n'
             '  void operator[]=(int index, $TYPE value) {\n'
             '    throw new UnsupportedOperationException("Cannot assign element of immutable List.");\n'
             '  }\n',
             TYPE=self._NarrowInputType(element_type))
 
     # TODO(sra): Use separate mixins for mutable implementations of List<T>.
     # TODO(sra): Use separate mixins for typed array implementations of List<T>.
     if self._interface.id != 'NodeList':
       template_file = 'immutable_list_mixin.darttemplate'
-      template = self._template_loader.Load(template_file)
+      has_contains = any(op.id == 'contains' for op in self._interface.operations)
+      template = self._template_loader.Load(
+          template_file,
+          {'DEFINE_CONTAINS': not has_contains})
       self._members_emitter.Emit(template, E=self._DartType(element_type))
 
   def AddAttribute(self, attribute, html_name, read_only):
     if self._HasCustomImplementation(attribute.id):
       return
 
     if attribute.id != html_name:
       self._AddAttributeUsingProperties(attribute, html_name, read_only)
       return
 
@@ skipping 399 matching lines @@
 
     library_emitter = self._multiemitter.FileEmitter(library_file_path)
     library_file_dir = os.path.dirname(library_file_path)
     auxiliary_dir = os.path.relpath(auxiliary_dir, library_file_dir)
     imports_emitter = library_emitter.Emit(
         self._template, AUXILIARY_DIR=massage_path(auxiliary_dir))
     for path in sorted(self._path_to_emitter.keys()):
       relpath = os.path.relpath(path, library_file_dir)
       imports_emitter.Emit(
           "#source('$PATH');\n", PATH=massage_path(relpath))