dart2js: add failing test for generic types.

tests/language/generic_instanceof4_test.dart

+// Copyright (c) 2014, 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.
+// Test that instanceof works correctly with type variables.
+
+import "package:expect/expect.dart";
+
+class A<T> {
+  foo(x) {
+    // Don't inline.
+    if (new DateTime.now().millisecondsSinceEpoch == 42) return foo(x);
+    return x is T;
+  }
+}
+
+class BB {}
+
+class B<T> implements BB {
+  foo() {
+    // Don't inline.
+    if (new DateTime.now().millisecondsSinceEpoch == 42) return foo(x);
+    return new A<T>().foo(new B());
+  }
+}
+
+main() {
+  Expect.isTrue(new B<BB>().foo());
+}