Make createTempDir() synchronous.

utils/pub/system_cache.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library system_cache;
 
 import 'dart:io';
 import 'dart:async';
 
 import 'git_source.dart';
@@ skipping 83 matching lines @@
         .whenComplete(() { _pendingInstalls.remove(id); });
     _pendingInstalls[id] = future;
     return future;
   }
 
   /// Create a new temporary directory within the system cache. The system
   /// cache maintains its own temporary directory that it uses to stage
   /// packages into while installing. It uses this instead of the OS's system
   /// temp directory to ensure that it's on the same volume as the pub system
   /// cache so that it can move the directory from it.
-  Future<Directory> createTempDir() {
-    return defer(() {
-      var temp = ensureDir(tempDir);
-      return io.createTempDir(join(temp, 'dir'));
-    });
+  Directory createTempDir() {
+    var temp = ensureDir(tempDir);
+    return io.createTempDir(join(temp, 'dir'));
   }
 
   /// Delete's the system cache's internal temp directory.
   Future deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     return defer(() {
       if (!dirExists(tempDir)) return;
       return deleteDir(tempDir);
     });
   }
 }