Migrate async tests to strong

tests/lib_strong/async/schedule_microtask3_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.
+
+library run_async_test;
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+  test("run async timer after async test", () {
+    // Check that Timers don't run before the async callbacks.
+    bool timerCallbackExecuted = false;
+
+    Timer.run(expectAsync(() { timerCallbackExecuted = true; }));
+
+    scheduleMicrotask(expectAsync(() {
+      Expect.isFalse(timerCallbackExecuted);
+    }));
+
+    scheduleMicrotask(expectAsync(() {
+      // Busy loop.
+      var sum = 1;
+      var sw = new Stopwatch()..start();
+      while (sw.elapsedMilliseconds < 5) {
+        sum++;
+      }
+      if (sum == 0) throw "bad";  // Just to use the result.
+      scheduleMicrotask(expectAsync(() {
+        Expect.isFalse(timerCallbackExecuted);
+      }));
+    }));
+
+    scheduleMicrotask(expectAsync(() {
+      Expect.isFalse(timerCallbackExecuted);
+    }));
+  });
+}