Support runtime check of function types.

tests/language/function_subtype_factory0_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 with type variables in factory constructors.
+
+import 'package:expect/expect.dart';
+
+typedef void Foo<T>(T t);
+
+class C<T> {
+  factory C(foo) {
+    if (foo is Foo<T>) {
+      return new C.internal();
+    }
+    return null;
+  }
+  C.internal();
+}
+
+void method(String s) {}
+
+void main() {
+  Expect.isNotNull(new C<String>(method));
+  Expect.isNull(new C<bool>(method));
+}