Migrate async tests to strong

tests/lib_strong/async/stream_first_where_test.dart

+// Copyright (c) 2011, 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.
+
+library stream_controller_async_test;
+
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'package:unittest/unittest.dart';
+import 'event_helper.dart';
+import 'stream_state_helper.dart';
+
+class A { const A(); }
+class B extends A { const B(); }
+
+main() {
+  Events sentEvents = new Events()..close();
+
+  // Make sure that firstWhere allows to return instances of types that are
+  // different than the generic type of the stream.
+  test("firstWhere with super class", () {
+    StreamController c = new StreamController<B>();
+    Future f = c.stream.firstWhere((x) => false, defaultValue: () => const A());
+    f.then(expectAsync((v) { Expect.equals(const A(), v); }));
+    sentEvents.replay(c);
+  });
+}