Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

tests/compiler/dart2js/mirror_private_name_inheritance_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 final fields in @MirrorsUsed are still inferred.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart' show runCompiler;
 import 'compiler_helper.dart' show findElement;
@@ skipping 25 matching lines @@
 };
 
 void main() {
   asyncTest(() async {
     var result = await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     var compiler = result.compiler;
 
     var superclass =
         findElement(compiler, 'Super', Uri.parse('memory:lib.dart'));
     var subclass = findElement(compiler, 'Subclass');
-    var oracle = compiler.backend.isAccessibleByReflection;
+    var oracle = compiler.backend.mirrorsData.isAccessibleByReflection;
     print(superclass.lookupMember('_private'));
     Expect.isTrue(oracle(superclass.lookupMember('_private')));
     Expect.isFalse(oracle(subclass.lookupMember('_private')));
   });
 }