Remove warnings from dart2js test-suite.

tests/compiler/dart2js/deferred_not_in_main_test.dart

 // Copyright (c) 2014, 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.
 
 // Test of the graph segmentation algorithm used by deferred loading
 // to determine which elements can be deferred and which libraries
 // much be included in the initial download (loaded eagerly).
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 22 matching lines @@
                                    handler.diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    [],
                                    {});
   asyncTest(() => compiler.run(Uri.parse('memory:main.dart')).then((_) {
     var main = compiler.mainApp.find(dart2js.Compiler.MAIN);
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
 
     var mainOutputUnit = compiler.deferredLoadTask.mainOutputUnit;
-    var classes = compiler.backend.emitter.neededClasses;
+    var backend = compiler.backend;
+    var classes = backend.emitter.neededClasses;
     var lib1 = compiler.libraries["memory:lib1.dart"];
     var lib2 = compiler.libraries["memory:lib2.dart"];
     var foo1 = lib1.find("foo1");
     var foo2 = lib2.find("foo2");
 
-    var outputClassLists = compiler.backend.emitter.outputClassLists;
+    var outputClassLists = backend.emitter.outputClassLists;
 
     Expect.notEquals(mainOutputUnit, outputUnitForElement(foo2));
   }));
 }
 
 // lib1 imports lib2 deferred. But mainlib never uses DeferredLibrary.
 // Test that this case works.
 const Map MEMORY_SOURCE_FILES = const {
   "main.dart":"""
 library mainlib;
@@ skipping 15 matching lines @@
 void foo1() {
   def.load().then((_) => lib2.foo2());
 }
 """,
   "lib2.dart":"""
 library lib2;
 
 void foo2() {}
 """,
 };