Remove warnings from dart2js test-suite.

tests/compiler/dart2js/mirror_tree_shaking_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 that tree-shaking hasn't been turned off.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart'
        show NullSink;
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart'
        show Diagnostic;
 
 import 'dart:async';
+import '../../../sdk/lib/_internal/compiler/implementation/js_backend/js_backend.dart'
+       show JavaScriptBackend;
 
 main() {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   var provider = new MemorySourceFileProvider(MEMORY_SOURCE_FILES);
   void diagnosticHandler(Uri uri, int begin, int end,
                          String message, Diagnostic kind) {
     if (kind == Diagnostic.VERBOSE_INFO
@@ skipping 15 matching lines @@
                                    packageRoot,
                                    [],
                                    {});
   asyncTest(() => compiler.run(Uri.parse('memory:main.dart')).then((_) {
     Expect.isFalse(compiler.compilationFailed);
     Expect.isFalse(compiler.enqueuer.resolution.hasEnqueuedEverything);
     Expect.isFalse(compiler.enqueuer.resolution.hasEnqueuedReflectiveStaticFields);
     Expect.isFalse(compiler.enqueuer.codegen.hasEnqueuedEverything);
     Expect.isFalse(compiler.enqueuer.codegen.hasEnqueuedReflectiveStaticFields);
     Expect.isFalse(compiler.disableTypeInference);
-    Expect.isFalse(compiler.backend.hasRetainedMetadata);
+    JavaScriptBackend backend = compiler.backend;
+    Expect.isFalse(backend.hasRetainedMetadata);
   }));
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   'main.dart': r"""
 import 'dart:mirrors';
 
 class Foo {
   noSuchMethod(invocation) {
     print('Invoked ${MirrorSystem.getName(invocation.memberName)}');
     return reflect('foobar').delegate(invocation);
   }
 }
 
 void main() {
   print(new Foo().substring(3));
 }
 """,
 };