Fix issue 11398 by making sure we look at mixin applications

tests/language/mixin_regress_11398_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";
+
+void main() {
+  var hva = new HasValueA();
+  hva.value = '42';
+  Expect.equals('42', hva.value);
+
+  var hvb = new HasValueB();
+  hvb.value = '87';
+  Expect.equals('87', hvb.value);
+
+  var hvc = new HasValueC();
+  hvc.value = '99';
+  Expect.equals('99', hvc.value);
+}
+
+abstract class Delegate {
+  String invoke(String value);
+}
+
+abstract class DelegateMixin {
+  String invoke(String value) => value;
+}
+
+abstract class HasValueMixin implements Delegate {
+  String _value;
+  set value(String value) { _value = invoke(value); }
+  String get value => _value;
+}
+
+class HasValueA extends Object with HasValueMixin, DelegateMixin {
+}
+
+class HasValueB extends Object with DelegateMixin, HasValueMixin {
+}
+
+class HasValueC extends Object with HasValueMixin {
+  String invoke(String value) => value;
+}