Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

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:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/js_backend/js_backend.dart' show JavaScriptBackend;
 import 'package:expect/expect.dart';
@@ skipping 11 matching lines @@
     Expect.isFalse(compiler.compilationFailed);
     Expect.isFalse(backend
         .mirrorsAnalysis.resolutionHandler.hasEnqueuedReflectiveElements);
     Expect.isFalse(backend
         .mirrorsAnalysis.resolutionHandler.hasEnqueuedReflectiveStaticFields);
     Expect.isFalse(
         backend.mirrorsAnalysis.codegenHandler.hasEnqueuedReflectiveElements);
     Expect.isFalse(backend
         .mirrorsAnalysis.codegenHandler.hasEnqueuedReflectiveStaticFields);
     Expect.isFalse(compiler.disableTypeInference);
-    Expect.isFalse(backend.hasRetainedMetadata);
+    Expect.isFalse(backend.mirrorsData.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));
 }
 """,
 };