Refactor visitSendSet for super compounds and assignment.

tests/compiler/dart2js/mock_compiler.dart

 // 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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 22 matching lines @@
     show DeferredLoadTask,
          OutputUnit;
 
 import 'mock_libraries.dart';
 
 class WarningMessage {
   Spannable node;
   Message message;
   WarningMessage(this.node, this.message);
 
-  toString() => message.toString();
+  toString() => message.kind.toString();
 }
 
 final Uri PATCH_CORE = new Uri(scheme: 'patch', path: 'core');
 
 class MockCompiler extends Compiler {
   api.DiagnosticHandler diagnosticHandler;
   List<WarningMessage> warnings;
   List<WarningMessage> errors;
   List<WarningMessage> hints;
   List<WarningMessage> infos;
@@ skipping 348 matching lines @@
   MockElement(Element enclosingElement)
       : super('', ElementKind.FUNCTION, Modifiers.EMPTY,
               enclosingElement);
 
   get node => null;
 
   parseNode(_) => null;
 
   bool get hasNode => false;
 }