Convert GeneratedContent to async and use AnalysisDriver for tasks graph.

pkg/analysis_server/tool/spec/codegen_matchers.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.
 
 /**
  * Code generation for the file "matchers.dart".
  */
 import 'dart:convert';
 
 import 'package:analyzer/src/codegen/tools.dart';
 import 'package:front_end/src/codegen/tools.dart';
 
 import 'api.dart';
 import 'from_html.dart';
 import 'implied_types.dart';
 import 'to_html.dart';
 
 final GeneratedFile target = new GeneratedFile(
-    'test/integration/support/protocol_matchers.dart', (String pkgPath) {
+    'test/integration/support/protocol_matchers.dart', (String pkgPath) async {
   CodegenMatchersVisitor visitor = new CodegenMatchersVisitor(readApi(pkgPath));
   return visitor.collectCode(visitor.visitApi);
 });
 
 class CodegenMatchersVisitor extends HierarchicalApiVisitor with CodeGenerator {
   /**
    * Visitor used to produce doc comments.
    */
   final ToHtmlVisitor toHtmlVisitor;
 
@@ skipping 152 matching lines @@
     for (TypeDecl choice in typeUnion.choices) {
       if (commaNeeded) {
         write(', ');
       }
       visitTypeDecl(choice);
       commaNeeded = true;
     }
     write('])');
   }
 }