Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

tests/compiler/dart2js/mirrors_used_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 the @MirrorsUsed annotation suppress hints and that only
 /// requested elements are retained for reflection.
 library dart2js.test.mirrors_used_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 91 matching lines @@
     Set recordedNames = new Set()
       ..addAll(fullEmitter.recordedMangledNames)
       ..addAll(fullEmitter.mangledFieldNames.keys)
       ..addAll(fullEmitter.mangledGlobalFieldNames.keys);
     Expect.setEquals(new Set.from(expectedNames), recordedNames);
 
     for (var library in compiler.libraryLoader.libraries) {
       library.forEachLocalMember((member) {
         if (library == compiler.mainApp && member.name == 'Foo') {
           Expect.isTrue(
-              compiler.backend.isAccessibleByReflection(member), '$member');
+              compiler.backend.mirrorsData.isAccessibleByReflection(member),
+              '$member');
           member.forEachLocalMember((classMember) {
             Expect.isTrue(
-                compiler.backend.isAccessibleByReflection(classMember),
+                compiler.backend.mirrorsData
+                    .isAccessibleByReflection(classMember),
                 '$classMember');
           });
         } else {
           Expect.isFalse(
-              compiler.backend.isAccessibleByReflection(member), '$member');
+              compiler.backend.mirrorsData.isAccessibleByReflection(member),
+              '$member');
         }
       });
     }
 
     int metadataCount = 0;
     Set<ConstantValue> compiledConstants = backend.constants.compiledConstants;
     // Make sure that most of the metadata constants aren't included in the
     // generated code.
     backend.processMetadata(compiler.enqueuer.resolution.processedEntities,
         (metadata) {
@@ skipping 48 matching lines @@
 library lib;
 
 import 'dart:mirrors';
 
 useReflect(type) {
   print(new Symbol('Foo'));
   print(MirrorSystem.getName(reflectClass(type).owner.qualifiedName));
 }
 """,
 };