Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/ssa/codegen.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:math' as math;
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
@@ skipping 2871 matching lines @@
       // function types.
       registry.registerTypeUse(
           new TypeUse.isCheck(compiler.commonElements.functionType));
     }
     registry.registerTypeUse(new TypeUse.isCheck(type));
 
     CheckedModeHelper helper;
     if (node.isBooleanConversionCheck) {
       helper = const CheckedModeHelper('boolConversionCheck');
     } else {
-      helper =
-          backend.getCheckedModeHelper(type, typeCast: node.isCastTypeCheck);
+      helper = backend.checkedModeHelpers
+          .getCheckedModeHelper(type, typeCast: node.isCastTypeCheck);
     }
 
     if (helper == null) {
       assert(type.isFunctionType);
       assert(node.usesMethodOnType);
 
       String name = node.isCastTypeCheck ? '_asCheck' : '_assertCheck';
       HInstruction reifiedType = node.inputs[0];
       HInstruction checkedInput = node.inputs[1];
       use(reifiedType);
@@ skipping 212 matching lines @@
     registry.registerStaticUse(new StaticUse.staticInvoke(
         helper, new CallStructure.unnamed(argumentCount)));
     return backend.emitter.staticFunctionAccess(helper);
   }
 
   @override
   void visitRef(HRef node) {
     visit(node.value);
   }
 }