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

tests/standalone/io/http_response_deadline_test.dart

Index: tests/standalone/io/http_response_deadline_test.dart
diff --git a/tests/standalone/io/http_response_deadline_test.dart b/tests/standalone/io/http_response_deadline_test.dart
index e10913ab076051b5e67acc04a7b1985e871e8919..5ec6a322f7f3923a2c159007041e8106c8d58031 100644
--- a/tests/standalone/io/http_response_deadline_test.dart
+++ b/tests/standalone/io/http_response_deadline_test.dart
@@ -59,6 +59,7 @@ void testDeadlineAndDetach(int connections) {
     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');