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_test.dart

+// 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";
+
+class A  {
+  foo(x, [y]) => '$x;$y';
+}
+
+class B extends A with M1, M2, M3 {
+}
+
+class M1 {}
+
+class M2 {
+  plain(x) => 'P $x';
+  bar(x, [y]) => '$y,$x';
+}
+
+class M3 {}
+
+makeB() {
+  return [new A(), new B()].last as B;
+}
+
+main() {
+  var b = makeB();
+  Expect.equals('1;2', b.foo(1,2));
+  Expect.equals('2;null', b.foo(2));
+  Expect.equals('P 3', b.plain(3));
+  Expect.equals('100,4', b.bar(4,100));
+  Expect.equals('null,5', b.bar(5));
+}