Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.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.
 
 library dart2js.js_emitter.program_builder;
 
 import '../../closure.dart' show ClosureFieldElement;
 import '../../common.dart';
 import '../../common/names.dart' show Names, Selectors;
 import '../../compiler.dart' show Compiler;
@@ skipping 525 matching lines @@
 
     List<StubMethod> noSuchMethodStubs = <StubMethod>[];
 
     if (backend.enabledNoSuchMethod && element.isObject) {
       Map<js.Name, Selector> selectors =
           classStubGenerator.computeSelectorsForNsmHandlers();
       selectors.forEach((js.Name name, Selector selector) {
         // If the program contains `const Symbol` names we have to retain them.
         String selectorName = selector.name;
         if (selector.isSetter) selectorName = "$selectorName=";
-        if (backend.symbolsUsed.contains(selectorName)) {
+        if (backend.mirrorsData.symbolsUsed.contains(selectorName)) {
           _symbolsMap[name] = selectorName;
         }
         noSuchMethodStubs.add(
             classStubGenerator.generateStubForNoSuchMethod(name, selector));
       });
     }
 
     if (element == helpers.closureClass) {
       // We add a special getter here to allow for tearing off a closure from
       // itself.
@@ skipping 91 matching lines @@
     }
     _classes[element] = result;
     return result;
   }
 
   bool _methodNeedsStubs(FunctionElement method) {
     return !method.functionSignature.optionalParameters.isEmpty;
   }
 
   bool _methodCanBeReflected(FunctionElement method) {
-    return backend.isAccessibleByReflection(method);
+    return backend.mirrorsData.isAccessibleByReflection(method);
   }
 
   bool _methodCanBeApplied(FunctionElement method) {
     return backend.hasFunctionApplySupport &&
         closedWorld.getMightBePassedToApply(method);
   }
 
   /* Map | List */ _computeParameterDefaultValues(FunctionSignature signature) {
     var /* Map | List */ optionalParameterDefaultValues;
     if (signature.optionalParametersAreNamed) {
@@ skipping 297 matching lines @@
       Constant constant = new Constant(name, holder, constantValue);
       _constants[constantValue] = constant;
     }
   }
 
   Holder _registerStaticStateHolder() {
     return _registry.registerHolder(namer.staticStateHolder,
         isStaticStateHolder: true);
   }
 }