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

sdk/lib/io/http.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.
 
 part of dart.io;
 
 /**
  * HTTP status codes.
  */
 abstract class HttpStatus {
@@ skipping 1207 matching lines @@
   static const int DEFAULT_HTTP_PORT = 80;
   static const int DEFAULT_HTTPS_PORT = 443;
 
   /**
    * Get and set the idle timeout of non-active persistent (keep-alive)
    * connections. The default value is 15 seconds.
    */
   Duration idleTimeout;
 
   /**
+   * Get and set the maximum number of live connections, to a single host.
+   *
+   * Increasing this number may lower performance and take up unwanted
+   * system resources.
+   *
+   * To disable, set to `null`.
+   *
+   * Default is `null`.
+   */
+  int maxConnectionsPerHost;
+
+  /**
    * Set and get the default value of the `User-Agent` header for all requests
    * generated by this [HttpClient]. The default value is
    * `Dart/<version> (dart:io)`.
    *
    * If the userAgent is set to `null`, no default `User-Agent` header will be
    * added to each request.
    */
   String userAgent;
 
   factory HttpClient() => new _HttpClient();
@@ skipping 675 matching lines @@
 class RedirectException implements HttpException {
   final String message;
   final List<RedirectInfo> redirects;
 
   const RedirectException(this.message, this.redirects);
 
   String toString() => "RedirectException: $message";
 
   Uri get uri => redirects.last.location;
 }