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

tests/standalone/io/http_response_deadline_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 41 matching lines @@
     }
     Future.wait(futures).then((_) => server.close());
   });
 }
 
 void testDeadlineAndDetach(int connections) {
   HttpServer.bind('localhost', 0).then((server) {
     server.listen((request) {
       request.response.deadline = const Duration(milliseconds: 0);
       request.response.contentLength = 5;
+      request.response.persistentConnection = false;
       request.response.detachSocket().then((socket) {
         new Timer(const Duration(milliseconds: 100), () {
           socket.write('stuff');
           socket.close();
           socket.listen(null);
         });
       });
     });
 
     var futures = [];
@@ skipping 12 matching lines @@
     Future.wait(futures).then((_) => server.close());
   });
 }
 
 void main() {
   testSimpleDeadline(10);
   testExceedDeadline(10);
   testDeadlineAndDetach(10);
 }