fix #29544, mixins to a class with named constructors

tests/language_strong/mixin_named_constructor_test.dart

+// Copyright (c) 2017, 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";
+
+var calls = <String>[];
+
+abstract class A {
+  bool _done = true;
+  var a = calls.add('A()') as dynamic;
+}
+
+abstract class B {
+  B.protected() {
+    calls.add('B.protected()');
+  }
+}
+
+class C extends B with A {
+  C() : super.protected() {
+    calls.add('C()');
+  }
+}
+
+void main() {
+  var c = new C();
+  print(calls.join(', '));
+  Expect.isTrue(c._done);
+  Expect.equals(calls.join(', '), 'A(), B.protected(), C()');
+}