Make StreamController's unnamed constructor create a single-sub stream.

pkg/http/test/mock_client_test.dart

Index: pkg/http/test/mock_client_test.dart
diff --git a/pkg/http/test/mock_client_test.dart b/pkg/http/test/mock_client_test.dart
index 02e713b00b6bb1d2abd1c59394609ac79e6b1932..6467351a2057e332e71e2ea897e4b6f63bcc1588 100644
--- a/pkg/http/test/mock_client_test.dart
+++ b/pkg/http/test/mock_client_test.dart
@@ -35,7 +35,7 @@ void main() {
   test('handles a streamed request', () {
     var client = new MockClient.streaming((request, bodyStream) {
       return bodyStream.bytesToString().then((bodyString) {
-        var stream = new StreamController<List<int>>.singleSubscription();
+        var stream = new StreamController<List<int>>();
         async.then((_) {
           stream.add('Request body was "$bodyString"'.charCodes);
           stream.close();