Migrate async tests to strong

tests/lib_strong/async/catch_errors20_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:async_helper/async_helper.dart';
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'catch_errors.dart';
+
+main() {
+  asyncStart();
+  Completer done = new Completer();
+
+  var events = [];
+  // Test that nested stream (from `catchErrors`) that is delayed by a future
+  // is waited for.
+  catchErrors(() {
+    catchErrors(() {
+      new Future.error(499);
+      new Future.delayed(const Duration(milliseconds: 20), () {
+        events.add(42);
+        done.complete(true);
+      });
+    }).listen(events.add,
+              onDone: () { events.add("done"); });
+    throw "foo";
+  }).listen((x) { events.add("outer: $x"); },
+            onDone: () { Expect.fail("Unexpected callback"); });
+
+  done.future.whenComplete(() {
+    // Give handlers time to run.
+    Timer.run(() {
+      Expect.listEquals(["outer: foo",
+                          499,
+                          42,
+                        ],
+                        events);
+      asyncEnd();
+    });
+  });
+}