fix mixins with factory constructors

tests/language_strong/mixin_factory_constructor_test.dart

Index: tests/language_strong/mixin_factory_constructor_test.dart
diff --git a/tests/language_strong/mixin_factory_constructor_test.dart b/tests/language_strong/mixin_factory_constructor_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ac0e22762d52ef281d4e0a5c3d60b25161fd370f
--- /dev/null
+++ b/tests/language_strong/mixin_factory_constructor_test.dart
@@ -0,0 +1,28 @@
+// 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";
+
+class Base {
+  var y;
+  Base._() {
+    y = "world";
+  }
+}
+
+abstract class Mixin implements Base {
+  final x = "hello";
+  factory Mixin() => new _MixinAndBase._();
+}
+
+// TODO(jmesserly): according to the spec, this does not appear to be a valid
+// mixin (because it declares a constructor), however it is supported by Dart
+// implementations.
+class _MixinAndBase = Base with Mixin;
+
+void main() {
+  var val = new Mixin();
+  Expect.equals(val.x, "hello");
+  Expect.equals(val.y, "world");
+}