Enable throttleing of HttpClient per host, with default being 6.

tests/standalone/io/http_shutdown_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
 
 import "package:expect/expect.dart";
@@ skipping 144 matching lines @@
                 request.response.done.catchError((e) {});
               },
               onError: (error) { });
         },
         onError: (error) { });
 
     // Create a number of client requests and keep then active. Then
     // close the client and wait for the server to lose all active
     // connections.
     var client= new HttpClient();
+    client.maxConnectionsPerHost = totalConnections;
     for (int i = 0; i < totalConnections; i++) {
       client.post("127.0.0.1", server.port, "/")
         .then((request) {
             request.add([0]);
             // TODO(sgjesse): Make this test work with
             //request.response instead of request.close() return
             //return request.response;
             request.done.catchError((e) {});
             return request.close();
           })
@@ skipping 23 matching lines @@
   test1(10);
   test2(1, 10);
   test2(10, 10);
   test2(10, 1000);
   test3(1);
   test3(10);
   test4();
   test5(1);
   test5(10);
 }