Add tests for `FutureOr<T>`.

tests/lib/async/future_or_bad_type_test.dart

Index: tests/lib/async/future_or_bad_type_test.dart
diff --git a/tests/lib/async/future_or_bad_type_test.dart b/tests/lib/async/future_or_bad_type_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6b11be0b62a111889ef06259db30c2871a2d5952
--- /dev/null
+++ b/tests/lib/async/future_or_bad_type_test.dart
@@ -0,0 +1,28 @@
+// Copyright (c) 2016, 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.
+
+// In non strong-mode, `FutureOr<T>` is dynamic, even if `T` doesn't exist.
+// `FutureOr<T>` can not be used as superclass, mixin, nor can it be
+// implemented (as interface).
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+
+class A
+    extends FutureOr<String>  /// extends: compile-time error
+    with FutureOr<bool>  /// with: compile-time error

[Lasse Reichstein Nielsen, 2016/12/08 18:20:28]

You need "extends Object with FutureOr<bool>". The "with" can't stand by itself.

[floitsch, 2016/12/08 18:41:55]

Done.

+    implements FutureOr<int>  /// implements: compile-time error
+{
+}
+
+main() {
+  Expect.isTrue(499 is FutureOr<A>);  /// 00: static type warning
+  Expect.isTrue(499 is FutureOr<Does<Not<Exist>>>);  /// 01: static type warning

[Lasse Reichstein Nielsen, 2016/12/08 18:20:28]

Hmm. What is this intended to do, and why? 
The "Does" is malformed, so it is "499 is FutureOr<Malformed>" aka "499 is
Malformed | 499 is Future<Malformed>". 
(That is: Document expected behavior, anything with malformed types is hard to
remember for most of us :)

[floitsch, 2016/12/08 18:41:55]

Added a comment.
  // FutureOr<T> should be treated like `dynamic`. Dynamically the `T` is
  // completely ignored. It can be a malformed type.

+
+  Expect.isTrue(499 is FutureOr<A>);  /// 00: static type warning
+  Expect.isTrue(499 is FutureOr<Does<Not<Exist>>>);  /// 01: static type warning
+
+  var a = new A();
+  Expect.isTrue(a.toString() is String);
+}