Convert GeneratedContent to async and use AnalysisDriver for tasks graph.

pkg/analyzer_plugin/tool/spec/to_html.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.
 
 /**
  * Code for displaying the API as HTML.  This is used both for generating a
  * full description of the API as a web page, and for generating doc comments
  * in generated code.
  */
 import 'dart:convert';
@@ skipping 110 matching lines @@
   -webkit-margin-before: 0;
   -webkit-margin-start: 0;
   -webkit-padding-start: 0;
 
   list-style-type: none;
 }
 '''
     .trim();
 
 final GeneratedFile target =
-    new GeneratedFile('doc/api.html', (String pkgPath) {
+    new GeneratedFile('doc/api.html', (String pkgPath) async {
   ToHtmlVisitor visitor = new ToHtmlVisitor(readApi(pkgPath));
   dom.Document document = new dom.Document();
   document.append(new dom.DocumentType('html', null, null));
   for (dom.Node node in visitor.collectHtml(visitor.visitApi)) {
     document.append(node);
   }
   return document.outerHtml;
 });
 
 String _toTitleCase(String str) {
@@ skipping 676 matching lines @@
     bool verticalBarNeeded = false;
     for (TypeDecl choice in typeUnion.choices) {
       if (verticalBarNeeded) {
         write(' | ');
       }
       visitTypeDecl(choice);
       verticalBarNeeded = true;
     }
   }
 }