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

sdk/lib/io/http.dart

Index: sdk/lib/io/http.dart
diff --git a/sdk/lib/io/http.dart b/sdk/lib/io/http.dart
index 21b177d84eaf5a41d1b6daf9e5904a4a2b41326f..b995137bb259b65c2fcf5a4bae56205a2c36ad56 100644
--- a/sdk/lib/io/http.dart
+++ b/sdk/lib/io/http.dart
@@ -1225,6 +1225,18 @@ abstract class HttpClient {
   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)`.