Make new StreamController be async by default.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index 8cfb2a3c1cc5f7ffec5f2fe1deb2df9709a2fcc5..0a16be90a452f51db8d237f26f90b4a22a9b8cd2 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -560,7 +560,8 @@ class _HttpOutboundConsumer implements StreamConsumer {
 
   _ensureController() {
     if (_controller != null) return;
-    _controller = new StreamController(onPause: () => _subscription.pause(),
+    _controller = new StreamController(sync: true,
+                                       onPause: () => _subscription.pause(),
                                        onResume: () => _subscription.resume(),
                                        onListen: () => _subscription.resume(),
                                        onCancel: _cancel);
@@ -1929,7 +1930,7 @@ class _HttpServer extends Stream<HttpRequest> implements HttpServer {
   // Set of currently connected clients.
   final Set<_HttpConnection> _connections = new Set<_HttpConnection>();
   final StreamController<HttpRequest> _controller
-      = new StreamController<HttpRequest>();
+      = new StreamController<HttpRequest>(sync: true);
 
   // TODO(ajohnsen): Use close queue?
 }