fix unsound cast failures in tests

tests/lib_strong/convert/streamed_conversion_json_decode1_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:convert';
 import "package:async_helper/async_helper.dart";
 
 final TESTS = [
@@ skipping 65 matching lines @@
       if (!isJsonEqual(o1[key], o2[key])) return false;
     }
     return true;
   }
   return false;
 }
 
 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();
   });
   return controller.stream.transform(decoder);
 }
 
 Stream<Object> decode(String str) {
   return createStream([str]);
 }
 
@@ skipping 23 matching lines @@
       [object, longString]
     ];
   });
   for (var test in TESTS) {
     var o = test[0];
     var string = test[1];
     checkIsJsonEqual(o, decode(string));
     checkIsJsonEqual(o, decode2(string));
   }
 }