Remove a stale TODO (#100)

Dart 2 provides strong guarantees that the reified type of an object
will be assignable to the static type, this isn't something we need a
runtime test for ourselves.
diff --git a/test/stream_splitter_test.dart b/test/stream_splitter_test.dart
index fd37f87..3748c5c 100644
--- a/test/stream_splitter_test.dart
+++ b/test/stream_splitter_test.dart
@@ -97,10 +97,6 @@
     expect(() => branch.listen(null), throwsStateError);
   });
 
-  // TODO(nweiz): Test that branches have the correct reified type once Dart
-  // 1.11 is released. In 1.10, the stream exposed by a StreamController didn't
-  // have a reified type.
-
   test('multiple branches each replay the stream', () async {
     var branch1 = splitter.split();
     controller.add(1);