Fix issue 10216. If the type mask is a subtype, and its base mixes in class A, it should consider a…

tests/language/mixin_issue10216_2_test.dart

Index: tests/language/mixin_issue10216_2_test.dart
===================================================================
--- tests/language/mixin_issue10216_2_test.dart	(revision 0)
+++ tests/language/mixin_issue10216_2_test.dart	(revision 0)
@@ -0,0 +1,20 @@
+// 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.
+
+import "package:expect/expect.dart";
+
+typedef M1 = Object with M0;
+typedef M2 = Object with M1;
+
+class M0 {
+  foo() => 42;
+}
+
+makeM2() {
+  return [new Object(), new M2()].last as M2;
+}
+
+main() {
+  Expect.equals(42, makeM2().foo());
+}