Convert GeneratedContent to async and use AnalysisDriver for tasks graph.

pkg/analyzer/tool/task_dependency_graph/check_test.dart

 // Copyright (c) 2015, 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.
 
 library analyzer.tool.task_dependency_graph.check_test;
 
 import 'dart:io';
 
 import 'package:front_end/src/codegen/tools.dart';
 import 'package:path/path.dart';
 
 import 'generate.dart';
 
 /**
  * Check that the target file has been code generated.  If it hasn't tell the
  * user to run generate.dart.
  */
-main() {
+main() async {
   String script = Platform.script.toFilePath(windows: Platform.isWindows);
   String pkgPath = normalize(join(dirname(script), '..', '..'));
-  GeneratedContent.checkAll(pkgPath, 'tool/task_dependency_graph/generate.dart',
-      <GeneratedContent>[target]);
+  await GeneratedContent.checkAll(pkgPath,
+      'tool/task_dependency_graph/generate.dart', <GeneratedContent>[target]);
 }