A selector whose type mask is nullable, applies to methods on the null implementation.

tests/language/null_method_test.dart

Index: tests/language/null_method_test.dart
===================================================================
--- tests/language/null_method_test.dart	(revision 0)
+++ tests/language/null_method_test.dart	(revision 0)
@@ -0,0 +1,18 @@
+// 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.
+
+// Regression test for dart2js that used to not compile null methods
+// in the presence of typed selectors.
+
+class C {
+  foo(s) {
+    return s.hashCode;
+  }
+}
+
+main() {
+  var c = new C();
+  Expect.isNotNull(c.foo('foo'));
+  Expect.isNotNull(c.foo(null));
+}