Make web_socket_error_test pass analyzer (include missing library).

tests/standalone/io/web_socket_error_test.dart

 // Copyright (c) 2013, 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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 library dart.io;
 
 import "dart:async";
 import "dart:io";
+import "dart:math";
 import "dart:typed_data";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:path/path.dart";
 
 part "../../../sdk/lib/io/crypto.dart";
 
 
 const String webSocketGUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
@@ skipping 72 matching lines @@
 
 
 main() {
   asyncStart();
   new SecurityConfiguration(secure: false).runTests();
   initializeSSL();
   new SecurityConfiguration(secure: true).runTests();
   asyncEnd();
 }