Add tests demonstrating bug #25605.

tests/language/mixin_of_mixin_test.dart

+// 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 text from section 12 ("Mixins") of the spec:
+//
+//     "A mixin application of the form S with M; defines a class C ...
+//     ... C declares the same instance members as M ..."
+//
+// This means that if M is itself a mixin application, then things
+// mixed into M are accessible through C.  But if M simply *extends* a
+// mixin application (e.g. because M is declared as `class M extends X
+// with Y { ... }`) then things mixed into the mixin application that
+// M extends are not accessible through C.
+
+class A { a() => null; }
+class B { b() => null; }
+class C { c() => null; }
+class D { d() => null; }
+
+// Note: by a slight abuse of syntax, `class M1 = A with B, C;` effectively
+// means `class M1 = (A with B) with C;`, therefore M1 declares c(), but it
+// merely inherits a() and b().
+class M1 = A with B, C; // declares c()
+
+class M2 extends M1 { m2() => null; }
+class M3 extends A with B, C { m3() => null; }
+class T1 = D with M1; // declares c()
+class T2 = D with M2; // declares m2()
+class T3 = D with M3; // declares m3()
+class T4 extends D with M1 {} // extends a class which declares c()
+class T5 extends D with M2 {} // extends a class which declares m2()
+class T6 extends D with M3 {} // extends a class which declares m3()
+
+main() {
+  new T1().a();  /// 01: static type warning, runtime error
+  new T1().b();  /// 02: static type warning, runtime error
+  new T1().c();  /// 03: ok
+  new T2().a();  /// 04: static type warning, runtime error
+  new T2().b();  /// 05: static type warning, runtime error
+  new T2().c();  /// 06: static type warning, runtime error
+  new T2().m2(); /// 07: ok
+  new T3().a();  /// 08: static type warning, runtime error
+  new T3().b();  /// 09: static type warning, runtime error
+  new T3().c();  /// 10: static type warning, runtime error
+  new T3().m3(); /// 11: ok
+  new T4().a();  /// 12: static type warning, runtime error
+  new T4().b();  /// 13: static type warning, runtime error
+  new T4().c();  /// 14: ok
+  new T5().a();  /// 15: static type warning, runtime error
+  new T5().b();  /// 16: static type warning, runtime error
+  new T5().c();  /// 17: static type warning, runtime error
+  new T5().m2(); /// 18: ok
+  new T6().a();  /// 19: static type warning, runtime error
+  new T6().b();  /// 20: static type warning, runtime error
+  new T6().c();  /// 21: static type warning, runtime error
+  new T6().m3(); /// 22: ok
+}