Make scheduled_test play nicer with plain unittest tests.

pkg/scheduled_test/test/unittest_compatibility_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.
+
+// This isn't a common use-case, but we want to make sure scheduled_test doesn't
+// step on unittest's toes too much.
+import 'dart:async';
+
+import 'package:scheduled_test/scheduled_test.dart' as scheduled_test;
+import 'package:unittest/unittest.dart' as unittest;
+
+void main() {
+  scheduled_test.test('scheduled_test throws', () {
+    scheduled_test.expect(new Future.error('foo'),
+        scheduled_test.throwsA('foo'));
+  });
+
+  unittest.test('unittest throws', () {
+    // We test a future matcher because scheduled_test modifies some of
+    // unittest's asynchrony infrastructure.
+    unittest.expect(new Future.error('foo'), unittest.throwsA('foo'));
+  });
+}