Convert GeneratedContent to async and use AnalysisDriver for tasks graph.

pkg/analyzer_plugin/tool/spec/codegen_protocol_constants.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 'api.dart';
 import 'codegen_dart.dart';
 import 'from_html.dart';
 
-final GeneratedFile target =
-    new GeneratedFile('lib/protocol/protocol_constants.dart', (String pkgPath) {
+final GeneratedFile target = new GeneratedFile(
+    'lib/protocol/protocol_constants.dart', (String pkgPath) async {
   CodegenVisitor visitor = new CodegenVisitor(readApi(pkgPath));
   return visitor.collectCode(visitor.visitApi);
 });
 
 /**
  * A visitor that produces Dart code defining constants associated with the API.
  */
 class CodegenVisitor extends DartCodegenVisitor with CodeGenerator {
   CodegenVisitor(Api api) : super(api) {
     codeGeneratorSettings.commentLineLength = 79;
@@ skipping 137 matching lines @@
     int index = first.indexOf(regExp, start);
     while (index >= 0) {
       components.add(first.substring(start - 1, index));
       start = index + 1;
       index = first.indexOf(regExp, start);
     }
     components.add(first.substring(start - 1));
     return components;
   }
 }