Add stack chain support to pkg/watcher and pkg/http.

pkg/http/lib/src/base_client.dart

Index: pkg/http/lib/src/base_client.dart
diff --git a/pkg/http/lib/src/base_client.dart b/pkg/http/lib/src/base_client.dart
index 86925c36fed345923a91a6cd2c4803aceb7a5012..87a4224ce9995867992db0eebed3a5949ee35401 100644
--- a/pkg/http/lib/src/base_client.dart
+++ b/pkg/http/lib/src/base_client.dart
@@ -128,7 +128,7 @@ abstract class BaseClient implements Client {
   /// Sends a non-streaming [Request] and returns a non-streaming [Response].
   Future<Response> _sendUnstreamed(String method, url,
       Map<String, String> headers, [body, Encoding encoding]) {
-    return new Future.sync(() {
+    return syncFuture(() {
       if (url is String) url = Uri.parse(url);
       var request = new Request(method, url);