Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

tests/compiler/dart2js/deferred_mirrors_test.dart

 // Copyright (c) 2014, 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 'dart:async';
 import 'package:expect/expect.dart';
@@ skipping 39 matching lines @@
     // Just check that the compile runs.
     // This is a regression test.
     Expect.isTrue(true);
   });
   await runTest('memory:main3.dart', (compiler) {
     var main = compiler.mainFunction;
     Expect.isNotNull(main, "Could not find 'main'");
     compiler.deferredLoadTask.onResolutionComplete(main);
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
 
-    Expect.isFalse(compiler.backend.hasInsufficientMirrorsUsed);
+    Expect.isFalse(compiler.backend.mirrorsData.hasInsufficientMirrorsUsed);
     var mainLib = lookupLibrary(compiler, "memory:main3.dart");
     var lib3 = lookupLibrary(compiler, "memory:lib3.dart");
     var C = mainLib.find("C");
     var foo = lib3.find("foo");
 
     Expect.notEquals(outputUnitForElement(main), outputUnitForElement(foo));
     Expect.equals(outputUnitForElement(main), outputUnitForElement(C));
   });
   await runTest('memory:main4.dart', (compiler) {
     var main = compiler.mainFunction;
@@ skipping 121 matching lines @@
 library lib5;
 
 foo() {}
 """,
   "lib6.dart": """
 library lib6;
 
 foo() {}
 """,
 };