Replace defer() with Future.of in pub.

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index 7848915628c84a57047b05ec5dddfccdf86b7653..0af650bda17e36943bffb7cebc344503f6bb4bbb 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -550,7 +550,7 @@ 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 defer(() {
+  return new Future.of(() {
     var tempDir = createTempDir();
     return new Future.of(() => fn(tempDir))
         .whenComplete(() => deleteEntry(tempDir));