Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart' show Resolution, ParsingContext;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_constructors.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../diagnostics/messages.dart' show MessageTemplate;
 import '../ordered_typeset.dart' show OrderedTypeSet;
 import '../resolution/class_members.dart' show ClassMemberMixin;
 import '../resolution/resolution.dart' show AnalyzableElementX;
 import '../resolution/scope.dart'
     show ClassScope, LibraryScope, Scope, TypeDeclarationScope;
 import '../resolution/tree_elements.dart' show TreeElements;
 import '../resolution/typedefs.dart' show TypedefCyclicVisitor;
 import '../script.dart';
-import 'package:front_end/src/fasta/scanner.dart'
-    show ErrorToken, Token;
-import 'package:front_end/src/fasta/scanner.dart' as Tokens
-    show EOF_TOKEN;
+import 'package:front_end/src/fasta/scanner.dart' show ErrorToken, Token;
+import 'package:front_end/src/fasta/scanner.dart' as Tokens show EOF_TOKEN;
 import '../tree/tree.dart';
 import '../util/util.dart';
 import 'common.dart';
 import 'elements.dart';
 import 'resolution_types.dart';
 import 'visitor.dart' show ElementVisitor;
 
 /// Object that identifies a declaration site.
 ///
 /// For most elements, this is the element itself, but for variable declarations
@@ skipping 3433 matching lines @@
       body = node.asFunctionExpression().body;
     }
     return new ParsedResolvedAst(
         declaration,
         node,
         body,
         definingElement.treeElements,
         definingElement.compilationUnit.script.resourceUri);
   }
 }