Convert GeneratedContent to async and use AnalysisDriver for tasks graph.

pkg/analysis_server/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;
@@ skipping 12 matching lines @@
   'abstract': '0x01',
   'const': '0x02',
   'final': '0x04',
   'static': '0x08',
   'private': '0x10',
   'deprecated': '0x20'
 };
 
 GeneratedFile target(bool responseRequiresRequestTime) {
   return new GeneratedFile('lib/protocol/protocol_generated.dart',
-      (String pkgPath) {
+      (String pkgPath) async {
     CodegenProtocolVisitor visitor = new CodegenProtocolVisitor(
         path.basename(pkgPath), responseRequiresRequestTime, readApi(pkgPath));
     return visitor.collectCode(visitor.visitApi);
   });
 }
 
 /**
  * Callback type used to represent arbitrary code generation.
  */
 typedef void CodegenCallback();
@@ skipping 1273 matching lines @@
 
   @override
   String get asClosure => '($type value) => ${callback('value')}';
 
   @override
   bool get isIdentity => false;
 
   @override
   String asSnippet(String value) => callback(value);
 }