Generate constants in server as we do in plugin

pkg/analysis_server/tool/spec/generate_all.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:io';
 
 import 'package:front_end/src/codegen/tools.dart';
 import 'package:path/path.dart';
 
 import 'codegen_analysis_server.dart' as codegen_analysis_server;
 import 'codegen_dart_protocol.dart' as codegen_dart_protocol;
 import 'codegen_inttest_methods.dart' as codegen_inttest_methods;
 import 'codegen_java_types.dart' as codegen_java_types;
 import 'codegen_matchers.dart' as codegen_matchers;
+import 'codegen_protocol_constants.dart' as codegen_protocol_constants;
 import 'to_html.dart' as to_html;
 
 /**
  * Generate all targets.
  */
 main() {
   String script = Platform.script.toFilePath(windows: Platform.isWindows);
   String pkgPath = normalize(join(dirname(script), '..', '..'));
   GeneratedContent.generateAll(pkgPath, allTargets);
 }
 
 /**
  * Get a list of all generated targets.
  */
 List<GeneratedContent> get allTargets {
   List<GeneratedContent> targets = <GeneratedContent>[];
   targets.add(codegen_analysis_server.target);
   targets.add(codegen_dart_protocol.target(false));
   targets.add(codegen_java_types.targetDir);
   targets.add(codegen_inttest_methods.target);
   targets.add(codegen_matchers.target);
+  targets.add(codegen_protocol_constants.target);
   targets.add(to_html.target);
   return targets;
 }