Add shared tests demonstrating bug #25614.

tests/language/mixin_supertype_subclass_test.dart

Index: tests/language/mixin_supertype_subclass_test.dart
diff --git a/tests/language/mixin_supertype_subclass_test.dart b/tests/language/mixin_supertype_subclass_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..70627f64d5140c264dde56f30aa8d55f22c5b9c8
--- /dev/null
+++ b/tests/language/mixin_supertype_subclass_test.dart
@@ -0,0 +1,30 @@
+// Copyright (c) 2016, 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.
+// SharedOptions=--supermixin
+
+// Validate the following test from section 12 ("Mixins") of the spec:
+//
+//     "Let M_A be a mixin derived from a class M with direct superclass
+//     S_static.
+//
+//     Let A be an application of M_A.  It is a static warning if the
+//     superclass of A is not a subtype of S_static."
+
+class B {}
+class C {}
+class D {}
+class E extends B with C implements D {}
+class F extends E {}
+class A = E with M;
+class M
+  extends B /// 01: ok
+  extends C /// 02: static type warning
+  extends D /// 03: ok
+  extends E /// 04: ok
+  extends F /// 05: static type warning
+  {}
+
+main() {
+  new A();
+}