Fix webdriver build warnings.

pkg/webdriver/lib/webdriver.dart

 // Copyright (c) 2011, 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 webdriver;
 
+import 'dart:async';
 import 'dart:json' as json;
 import 'dart:uri';
 import 'dart:io';
 
 part 'src/base64decoder.dart';
 
 /**
  * WebDriver bindings for Dart.
  *
  * These bindings are based on the WebDriver JSON wire protocol spec
@@ skipping 1332 matching lines @@
    * Get the log for a given log type. Log buffer is reset after each request.
    * Each log entry is a [Map] with these fields:
    *
    * 'timestamp' (int) - The timestamp of the entry.
    * 'level' (String) - The log level of the entry, for example, "INFO".
    * 'message' (String) - The log message.
    */
   Future<List<Map>> getLogs(String type) =>
       _post('log', params: { 'type': type });
 }