Refactor Future constructors.

tools/dom/docs/lib/docs.dart

Index: tools/dom/docs/lib/docs.dart
diff --git a/tools/dom/docs/lib/docs.dart b/tools/dom/docs/lib/docs.dart
index 1b38913ed9eaedccf757f8d74cb93f1b68823016..fa46c20d2cced593ad4b0d7c6edfdc1510394dc0 100644
--- a/tools/dom/docs/lib/docs.dart
+++ b/tools/dom/docs/lib/docs.dart
@@ -64,7 +64,7 @@ Future<bool> convert(Path libPath, Path 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);