Refactor Future constructors.

tools/dom/docs/lib/docs.dart

 // Copyright (c) 2013, 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.
 
 /**
  * A library for extracting the documentation from the various HTML libraries
  * ([dart:html], [dart:svg], [dart:web_audio], [dart:indexed_db]) and saving
  * those documentation comments to a JSON file.
  */
 
@@ skipping 46 matching lines @@
   }
 
   return analyze(paths, libPath, options: ['--preserve-comments'])
     .then((MirrorSystem mirrors) {
       var convertedJson = _generateJsonFromLibraries(mirrors);
       return _exportJsonToFile(convertedJson, jsonPath);
     });
 }
 
 Future<bool> _exportJsonToFile(Map convertedJson, Path jsonPath) {
-  return new Future.of(() {
+  return new Future.sync(() {
     final jsonFile = new File.fromPath(jsonPath);
     var writeJson = prettySerialize(convertedJson);
 
     var outputStream = jsonFile.openWrite();
     outputStream.writeln(writeJson);
     outputStream.close();
     return outputStream.done.then((_) => false);
   });
 }
 
@@ skipping 106 matching lines @@
     for (var s in tags.reflectee.split(',')) {
       domNames.add(s.trim());
     }
 
     if (domNames.length == 1 && domNames[0] == 'none') return <String>[];
     return domNames;
   } else {
     return <String>[];
   }
 }