Remove warnings from dart2js test-suite.

tests/compiler/dart2js/deferred_load_graph_segmentation_test.dart

 // Copyright (c) 2013, 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 25 matching lines @@
                                    [],
                                    {});
   asyncTest(() => compiler.run(Uri.parse('memory:main.dart')).then((_) {
     var main = compiler.mainApp.find(dart2js.Compiler.MAIN);
     Expect.isNotNull(main, "Could not find 'main'");
     compiler.deferredLoadTask.onResolutionComplete(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 inputElement = classes.where((e) => e.name == 'InputElement').single;
     var lib1 = compiler.libraries["memory:lib1.dart"];
     var foo1 = lib1.find("foo1");
     var lib2 = compiler.libraries["memory:lib2.dart"];
     var foo2 = lib2.find("foo2");
     var lib3 = compiler.libraries["memory:lib3.dart"];
     var foo3 = lib3.find("foo3");
     var lib4 = compiler.libraries["memory:lib4.dart"];
     var bar1 = lib4.find("bar1");
     var bar2 = lib4.find("bar2");
-    var outputClassLists = compiler.backend.emitter.outputClassLists;
+    var outputClassLists = backend.emitter.outputClassLists;
 
     Expect.equals(mainOutputUnit, outputUnitForElement(main));
     Expect.notEquals(mainOutputUnit, outputUnitForElement(foo1));
     Expect.notEquals(outputUnitForElement(foo1), outputUnitForElement(foo3));
     Expect.notEquals(outputUnitForElement(foo2), outputUnitForElement(foo3));
     Expect.notEquals(outputUnitForElement(foo1), outputUnitForElement(foo2));
     Expect.notEquals(outputUnitForElement(bar1), outputUnitForElement(bar2));
     // InputElement is native, so it should not appear on a classList
     Expect.isFalse(outputClassLists[outputUnitForElement(inputElement)]
         .contains(inputElement));
@@ skipping 109 matching lines @@
 
 bar1() {
   return "hello";
 }
 
 bar2() {
   return 2;
 }
 """,
 };