Add a test and improve comment on generic checks.

tests/language/generic_is_check_test.dart

Index: tests/language/generic_is_check_test.dart
===================================================================
--- tests/language/generic_is_check_test.dart	(revision 0)
+++ tests/language/generic_is_check_test.dart	(revision 0)
@@ -0,0 +1,13 @@
+// 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.
+
+class A<T> {
+  foo() => this is A<int>;
+}
+
+main() {
+  Expect.isTrue(new A().foo());
+  Expect.isTrue(new A<int>().foo());
+  Expect.isFalse(new A<String>().foo());
+}