Fix deprecation hints

pkg/analysis_server/lib/driver.dart

 // Copyright (c) 2014, 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 driver;
 
 import 'dart:io';
 
 import 'package:analysis_server/http_server.dart';
 import 'package:analysis_server/src/socket_server.dart';
@@ skipping 122 matching lines @@
     });
   }
 
   /**
    * Print information about how to use the server.
    */
   void _printUsage(ArgParser parser) {
     print('Usage: $BINARY_NAME [flags]');
     print('');
     print('Supported flags are:');
-    print(parser.getUsage());
+    print(parser.usage);
   }
 }