Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/closure.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.
 
 library closureToClassMapper;
 
 import 'common/names.dart' show Identifiers;
 import 'common/resolution.dart' show ParsingContext, Resolution;
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
@@ skipping 1054 matching lines @@
 
   ClosureClassMap globalizeClosure(
       FunctionExpression node, LocalFunctionElement element) {
     String closureName = computeClosureName(element);
     ClosureClassElement globalizedElement =
         new ClosureClassElement(node, closureName, compiler, element);
     // Extend [globalizedElement] as an instantiated class in the closed world.
     closedWorldRefiner.registerClosureClass(globalizedElement);
     FunctionElement callElement = new SynthesizedCallMethodElementX(
         Identifiers.call, element, globalizedElement, node, elements);
-    backend.maybeMarkClosureAsNeededForReflection(
+    backend.mirrorsData.maybeMarkClosureAsNeededForReflection(
         globalizedElement, callElement, element);
     MemberElement enclosing = element.memberContext;
     enclosing.nestedClosures.add(callElement);
     globalizedElement.addMember(callElement, reporter);
     globalizedElement.computeAllClassMembers(compiler.resolution);
     // The nested function's 'this' is the same as the one for the outer
     // function. It could be [null] if we are inside a static method.
     ThisLocal thisElement = closureData.thisLocal;
 
     return new ClosureClassMap(
@@ skipping 126 matching lines @@
 ///
 /// Move the below classes to a JS model eventually.
 ///
 abstract class JSEntity implements Entity {
   Entity get declaredEntity;
 }
 
 abstract class PrivatelyNamedJSEntity implements JSEntity {
   Entity get rootOfScope;
 }