Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/diagnostics/messages.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The messages in this file should meet the following guide lines:
  *
  * 1. The message should be a complete sentence starting with an uppercase
  * letter, and ending with a period.
  *
@@ skipping 44 matching lines @@
  * or ERROR about the duplicated element, and then report an INFO about the
  * location of the existing element.
  *
  * Generally, we want to provide messages that consists of three sentences:
  * 1. what is wrong, 2. why is it wrong, 3. how do I fix it. However, we
  * combine the first two in [template] and the last in [howToFix].
  */
 
 library dart2js.messages;
 
-import 'package:front_end/src/fasta/scanner.dart'
-    show ErrorToken, Token;
+import 'package:front_end/src/fasta/scanner.dart' show ErrorToken, Token;
 import 'generated/shared_messages.dart' as shared_messages;
 import 'invariant.dart' show invariant;
 import 'spannable.dart' show CURRENT_ELEMENT_SPANNABLE;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /// Keys for the [MessageTemplate]s.
 enum MessageKind {
   ABSTRACT_CLASS_INSTANTIATION,
   ABSTRACT_GETTER,
@@ skipping 3731 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }