Eliminate dependencies on pkg/front_end.

pkg/analyzer_plugin/tool/spec/codegen_protocol_common.dart

 // Copyright (c) 2017, 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:analyzer/src/codegen/tools.dart';
-import 'package:front_end/src/codegen/tools.dart';
 import 'package:path/path.dart' as path;
 
 import 'api.dart';
 import 'codegen_dart_protocol.dart';
 import 'from_html.dart';
 import 'implied_types.dart';
 
 GeneratedFile target(bool responseRequiresRequestTime) =>
     new GeneratedFile('lib/protocol/protocol_common.dart', (String pkgPath) {
       CodegenCommonVisitor visitor = new CodegenCommonVisitor(
@@ skipping 31 matching lines @@
   List<ImpliedType> getClassesToEmit() {
     List<ImpliedType> types = impliedTypes.values.where((ImpliedType type) {
       ApiNode node = type.apiNode;
       return node is TypeDefinition && node.isExternal;
     }).toList();
     types.sort((first, second) =>
         capitalize(first.camelName).compareTo(capitalize(second.camelName)));
     return types;
   }
 }