Fix static type errors in a number of tests

tests/standalone/io/socket_many_connections_test.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.
 //
 // Test creating a large number of socket connections.
 library ServerTest;
 
 import "package:expect/expect.dart";
+import "dart:async";
 import "dart:io";
 import "dart:isolate";
 part "testing_server.dart";
 
 const CONNECTIONS = 200;
 
 class SocketManyConnectionsTest {
 
   SocketManyConnectionsTest.start()
       : _receivePort = new ReceivePort(),
@@ skipping 74 matching lines @@
         onDone: closeHandler,
         onError: errorHandler);
   }
 
   int _connections = 0;
 }
 
 main() {
   SocketManyConnectionsTest test = new SocketManyConnectionsTest.start();
 }