Convert GeneratedContent to async and use AnalysisDriver for tasks graph.

pkg/analyzer_plugin/tool/spec/codegen_inttest_methods.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 "integration_test_methods.dart".
  */
 import 'dart:convert';
 
 import 'package:analyzer/src/codegen/tools.dart';
 import 'package:path/path.dart' as path;
 
 import 'api.dart';
 import 'codegen_dart.dart';
 import 'from_html.dart';
 import 'to_html.dart';
 
-final GeneratedFile target = new GeneratedFile(
-    'test/integration/support/integration_test_methods.dart', (String pkgPath) {
+final GeneratedFile target =
+    new GeneratedFile('test/integration/support/integration_test_methods.dart',
+        (String pkgPath) async {
   CodegenInttestMethodsVisitor visitor = new CodegenInttestMethodsVisitor(
       path.basename(pkgPath), readApi(pkgPath));
   return visitor.collectCode(visitor.visitApi);
 });
 
 /**
  * Visitor that generates the code for integration_test_methods.dart
  */
 class CodegenInttestMethodsVisitor extends DartCodegenVisitor
     with CodeGenerator {
@@ skipping 234 matching lines @@
         writeln('ResponseDecoder decoder = new ResponseDecoder($kind);');
         writeln("return new $resultClass.fromJson(decoder, 'result', result);");
       } else {
         writeln('outOfTestExpect(result, isNull);');
         writeln('return null;');
       }
     });
     writeln('}');
   }
 }