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..edb614b654ef60d49d2f536ee8c28506f4a02355 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 6.

[Søren Gjesse, 2014/06/02 10:33:20]

Make the default `null` to keep the existing behavior unless you ask for
something else.

[Anders Johnsen, 2014/06/02 10:38:46]

I was under the assumption we wanted to provide a 'nice behaving' default
behavior, but I've disabled it by default.

Come to think of it, if it's not default, we might as well have put it into a
package.

+   */
+  int maxConnectionsPerHost = 6;
+
+  /**
    * 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)`.