Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/parser/diet_parser_task.dart

 // Copyright (c) 2015, 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.parser.diet.task;
 
 import '../common.dart';
 import '../common/backend_api.dart' show Backend;
 import '../common/tasks.dart' show CompilerTask, Measurer;
 import '../elements/elements.dart' show CompilationUnitElement;
 import '../id_generator.dart';
 import 'package:front_end/src/fasta/scanner.dart' show Token;
 import 'element_listener.dart' show ElementListener, ScannerOptions;
 import 'package:front_end/src/fasta/parser.dart'
     show Listener, ParserError, TopLevelParser;
 
 class PartialParser extends TopLevelParser {
-  PartialParser(Listener listener)
-      : super(listener);
+  PartialParser(Listener listener) : super(listener);
 
   Token parseFormalParameters(Token token) => skipFormalParameters(token);
 }
 
 class DietParserTask extends CompilerTask {
   final IdGenerator _idGenerator;
   final Backend _backend;
   final DiagnosticReporter _reporter;
 
   DietParserTask(
@@ skipping 12 matching lines @@
       try {
         parser.parseUnit(tokens);
       } on ParserError catch (_) {
         // TODO(johnniwinther): assert that the error was reported once there is
         // a [hasErrorBeenReported] field in [DiagnosticReporter]
         // The error should have already been reported by the parser.
       }
     });
   }
 }