Migrate async tests to strong

tests/lib_strong/async/stream_periodic6_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.
+
+// Test merging streams.
+library dart.test.stream_from_iterable;
+
+import "dart:async";
+import 'package:unittest/unittest.dart';
+
+main() {
+  test("stream-periodic1", () {
+    Stream stream = new Stream.periodic(const Duration(milliseconds: 1),
+                                        (i) {
+                                          if (i == 3) throw 42;
+                                          return i;
+                                        });
+    int expected = 0;
+    var subscription;
+    subscription = stream.listen(expectAsync((data) {
+      expect(data, expected++);
+      if (expected == 5) subscription.cancel();
+    }, count: 4),
+    onError: expectAsync((e, s) {
+      expect(e, 42);
+      expected++;
+    }));
+  });
+}