Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/ssa/builder.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:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
 import '../closure.dart';
 import '../common.dart';
@@ skipping 2713 matching lines @@
     }
     ast.LiteralString string = argument.asLiteralString();
     if (string == null) {
       reporter.reportErrorMessage(argument, MessageKind.GENERIC,
           {'text': 'Error: Expected a literal string.'});
     }
     String name = string.dartString.slowToString();
     bool value = false;
     switch (name) {
       case 'MUST_RETAIN_METADATA':
-        value = backend.mustRetainMetadata;
+        value = backend.mirrorsData.mustRetainMetadata;
         break;
       case 'USE_CONTENT_SECURITY_POLICY':
         value = compiler.options.useContentSecurityPolicy;
         break;
       default:
         reporter.reportErrorMessage(node, MessageKind.GENERIC,
             {'text': 'Error: Unknown internal flag "$name".'});
     }
     stack.add(graph.addConstantBool(value, closedWorld));
   }
@@ skipping 3986 matching lines @@
       this.oldReturnLocal,
       this.oldReturnType,
       this.oldResolvedAst,
       this.oldStack,
       this.oldLocalsHandler,
       this.inTryStatement,
       this.allFunctionsCalledOnce,
       this.oldElementInferenceResults)
       : super(function);
 }