Refactor BackendHelpers to be reusageable with KernelWorldBuilder

tests/compiler/dart2js/scanner_test.dart

 // Copyright (c) 2012, 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 "package:expect/expect.dart";
 import 'package:front_end/src/fasta/scanner.dart';
 import 'package:front_end/src/fasta/scanner/characters.dart';
 import 'package:front_end/src/fasta/scanner/precedence.dart';
 import 'dart:typed_data';
 
-
 Token scan(List<int> bytes) {
   List<int> zeroTerminated = new Uint8List(bytes.length + 1);
   zeroTerminated.setRange(0, bytes.length, bytes);
   zeroTerminated[bytes.length] = 0;
   return new Utf8BytesScanner(zeroTerminated).tokenize();
 }
 
 Token scanUTF8(List<int> bytes) {
   int l = bytes.length;
   List<int> stringLiteral = new Uint8List(l + 3);
@@ skipping 180 matching lines @@
     0x20,
     0x57,
     0x6f,
     0x72,
     0x6c,
     0x64,
     0x21
   ]);
   Expect.equals(token.info, EOF_INFO); // Treated as a comment.
 }