Replace defer() with Future.of in pub.

utils/pub/pub.dart

Index: utils/pub/pub.dart
diff --git a/utils/pub/pub.dart b/utils/pub/pub.dart
index 5f52a53a7af28ad8fe1849c0bc0a0962ab810f22..811c198de0e2b2537d740318313a28ba5a21d425 100644
--- a/utils/pub/pub.dart
+++ b/utils/pub/pub.dart
@@ -145,7 +145,7 @@ main() {
 /// Checks that pub is running on a supported platform. If it isn't, it prints
 /// an error message and exits. Completes when the validation is done.
 Future validatePlatform() {
-  return defer(() {
+  return new Future.of(() {
     if (Platform.operatingSystem != 'windows') return;
 
     return runProcess('ver', []).then((result) {
@@ -265,7 +265,7 @@ abstract class PubCommand {
       exit(_chooseExitCode(error));
     }
 
-    defer(() {
+    new Future.of(() {
       if (requiresEntrypoint) {
         // TODO(rnystrom): Will eventually need better logic to walk up
         // subdirectories until we hit one that looks package-like. For now,