Avoid creating invalid TypeMask.nonNullExact

tests/language/super_mixin_test.dart

Index: tests/language/super_mixin_test.dart
diff --git a/tests/language/super_mixin_test.dart b/tests/language/super_mixin_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..8263a1b6bd926c3b6f53e73998392d8fe8f0c0dd
--- /dev/null
+++ b/tests/language/super_mixin_test.dart
@@ -0,0 +1,25 @@
+// Copyright (c) 2015, 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.
+
+// Dart2js regression test: Test that the type mask for non-null exact Mixin is
+// created for Mixin.getter.
+
+import 'package:expect/expect.dart';
+
+class Mixin {
+  @NoInline()
+  get getter => 42;
+}
+
+class Superclass {
+
+}
+
+class Subclass extends Superclass with Mixin {
+  method() => super.getter;
+}
+
+void main() {
+  Expect.equals(42, new Subclass().method());
+}