Refactor Future constructors.

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index 697e02bd8c648e3438ee021203cee51274c5b855..32c4574d9102d22972d491c170d8f1573ca16c0e 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -550,9 +550,9 @@ Future timeout(Future input, int milliseconds, String description) {
 /// Returns a future that completes to the value that the future returned from
 /// [fn] completes to.
 Future withTempDir(Future fn(String path)) {
-  return new Future.of(() {
+  return new Future.sync(() {
     var tempDir = createTempDir();
-    return new Future.of(() => fn(tempDir))
+    return new Future.sync(() => fn(tempDir))
         .whenComplete(() => deleteEntry(tempDir));
   });
 }