Support runtime check of function types.

tests/language/function_subtype_cast0_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.
+// Dart test program for constructors and initializers.
+
+// Check function subtyping casts.
+
+import 'package:expect/expect.dart';
+
+typedef void Foo<T>(T t);
+typedef void Bar(int i);
+
+void bar(int i) {}
+
+void main() {
+  Expect.isNotNull(bar as Foo);
+  Expect.throws(() => bar as Foo<bool>, (e) => true);
+  Expect.isNotNull(bar as Foo<int>);
+  Expect.isNotNull(bar as Bar);
+}