Add more factory methods to FrontEndStrategy.

pkg/compiler/lib/src/js_backend/mirrors_data.dart

 // Copyright (c) 2017, 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 '../closure.dart';
 import '../common.dart';
 import '../common_elements.dart';
 import '../compiler.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart';
@@ skipping 189 matching lines @@
   /// element must be retained.
   final Set<ClassEntity> metaTargetsUsed = new Set<ClassEntity>();
 
   // TODO(johnniwinther): Avoid the need for this.
   final Compiler _compiler;
 
   final CompilerOptions _options;
 
   final CommonElements _commonElements;
 
-  final JavaScriptConstantCompiler _constants;
+  MirrorsDataImpl(this._compiler, this._options, this._commonElements);
 
-  MirrorsDataImpl(
-      this._compiler, this._options, this._commonElements, this._constants);
+  JavaScriptConstantCompiler get _constants => _compiler.backend.constants;
 
   void registerUsedMember(MemberElement member) {
     if (member == _commonElements.disableTreeShakingMarker) {
       isTreeShakingDisabled = true;
     } else if (member == _commonElements.preserveNamesMarker) {
       mustPreserveNames = true;
     } else if (member == _commonElements.preserveMetadataMarker) {
       mustRetainMetadata = true;
     } else if (member == _commonElements.preserveUrisMarker) {
       if (_options.preserveUris) mustPreserveUris = true;
@@ skipping 466 matching lines @@
   }
 
   /// Called when `const Symbol(name)` is seen.
   void registerConstSymbol(String name) {
     symbolsUsed.add(name);
     if (name.endsWith('=')) {
       symbolsUsed.add(name.substring(0, name.length - 1));
     }
   }
 }