Get rid of unneeded Future.immediate() calls.

utils/pub/pub.dart

Index: utils/pub/pub.dart
diff --git a/utils/pub/pub.dart b/utils/pub/pub.dart
index ac700f9b0de439d4c1e31ad157f806abaa6e4e62..adfcb8360de02bc51d6d372eaabe596518d1b906 100644
--- a/utils/pub/pub.dart
+++ b/utils/pub/pub.dart
@@ -246,16 +246,14 @@ abstract class PubCommand {
       this.entrypoint = entrypoint;
       try {
         var commandFuture = onRun();
-        if (commandFuture == null) return new Future.immediate(true);
+        if (commandFuture == null) return true;
 
         return commandFuture;
       } catch (error, trace) {
         handleError(error, trace);
-        return new Future.immediate(null);
       }
     });
 
-
     future
       .then((_) => cache_.deleteTempDir())
       .catchError((asyncError) {