Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/native/scanner.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.
 
 import '../common.dart';
 import '../parser/element_listener.dart' show ElementListener;
-import 'package:front_end/src/fasta/scanner.dart'
-    show BeginGroupToken, Token;
+import 'package:front_end/src/fasta/scanner.dart' show BeginGroupToken, Token;
 import 'package:front_end/src/fasta/scanner/token_constants.dart' as Tokens
     show STRING_TOKEN;
 
 void checkAllowedLibrary(ElementListener listener, Token token) {
   if (listener.scannerOptions.canUseNative) return;
   listener.reportErrorFromToken(token, MessageKind.NATIVE_NOT_SUPPORTED);
 }
 
 Token handleNativeBlockToSkip(ElementListener listener, Token token) {
   checkAllowedLibrary(listener, token);
@@ skipping 18 matching lines @@
     hasExpression = true;
     listener.beginLiteralString(token);
     listener.endLiteralString(0);
     token = token.next;
   }
   listener.endReturnStatement(hasExpression, begin, token);
   // TODO(ngeoffray): expect a ';'.
   // Currently there are method with both native marker and Dart body.
   return token.next;
 }