Revert "Use native async/await support in pub."

sdk/lib/_internal/pub_generated/lib/src/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 pub.system_cache;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
@@ skipping 88 matching lines @@
     var temp = ensureDir(tempDir);
     return io.createTempDir(temp, 'dir');
   }
 
   /// Deletes the system cache's internal temp directory.
   void deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     if (dirExists(tempDir)) deleteEntry(tempDir);
   }
 }