fix unsound cast failures in tests

tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart

Index: tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart
diff --git a/tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart b/tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart
index 2f7c5624f789f123a4b2b601b534825e24885907..88b4961152236be8d5d252f09827825726c15181 100644
--- a/tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart
+++ b/tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart
@@ -83,7 +83,7 @@ bool isJsonEqual(o1, o2) {
 Stream<Object> createStream(List<String> chunks) {
   var decoder = new JsonDecoder(null);
   var controller;
-  controller = new StreamController(onListen: () {
+  controller = new StreamController<String>(onListen: () {
     chunks.forEach(controller.add);
     controller.close();
   });