Eliminate dependencies on pkg/front_end.

pkg/analyzer_plugin/tool/spec/codegen_dart_protocol.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 'dart:convert';
 
 import 'package:analyzer/src/codegen/tools.dart';
-import 'package:front_end/src/codegen/tools.dart';
 import 'package:html/dom.dart' as dom;
 import 'package:path/path.dart' as path;
 
 import 'api.dart';
 import 'codegen_dart.dart';
 import 'from_html.dart';
 import 'implied_types.dart';
 import 'to_html.dart';
 
 /**
@@ skipping 1298 matching lines @@
 
   @override
   String get asClosure => '($type value) => ${callback('value')}';
 
   @override
   bool get isIdentity => false;
 
   @override
   String asSnippet(String value) => callback(value);
 }