Add *IndexSetIfNull methods to SemanticSendVisitor.

tests/compiler/dart2js/semantic_visitor_test.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.semantics_visitor_test;
 
 import 'dart:async';
 import 'dart:mirrors';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 656 matching lines @@
 
   VISIT_INDEX_SET,
   VISIT_COMPOUND_INDEX_SET,
   VISIT_SUPER_INDEX_SET,
   VISIT_UNRESOLVED_SUPER_INDEX_SET,
   VISIT_SUPER_COMPOUND_INDEX_SET,
   VISIT_UNRESOLVED_SUPER_COMPOUND_INDEX_SET,
   VISIT_UNRESOLVED_SUPER_GETTER_COMPOUND_INDEX_SET,
   VISIT_UNRESOLVED_SUPER_SETTER_COMPOUND_INDEX_SET,
 
+  VISIT_INDEX_SET_IF_NULL,
+  VISIT_SUPER_INDEX_SET_IF_NULL,
+  VISIT_UNRESOLVED_SUPER_INDEX_SET_IF_NULL,
+  VISIT_UNRESOLVED_SUPER_GETTER_INDEX_SET_IF_NULL,
+  VISIT_UNRESOLVED_SUPER_SETTER_INDEX_SET_IF_NULL,
+
   VISIT_LOGICAL_AND,
   VISIT_LOGICAL_OR,
   VISIT_IS,
   VISIT_IS_NOT,
   VISIT_AS,
 
   VISIT_CONST_CONSTRUCTOR_INVOKE,
   VISIT_BOOL_FROM_ENVIRONMENT_CONSTRUCTOR_INVOKE,
   VISIT_INT_FROM_ENVIRONMENT_CONSTRUCTOR_INVOKE,
   VISIT_STRING_FROM_ENVIRONMENT_CONSTRUCTOR_INVOKE,
@@ skipping 65 matching lines @@
   ERROR_INVALID_INDEX_SET,
   ERROR_INVALID_COMPOUND_INDEX_SET,
   ERROR_INVALID_INDEX_PREFIX,
   ERROR_INVALID_INDEX_POSTFIX,
 
   VISIT_CONSTANT_GET,
   VISIT_CONSTANT_INVOKE,
 
   PREVISIT_DEFERRED_ACCESS,
 }