Use library prefixes in scope of the mixin class when compiling a functions of

tests/language/mixin_prefix_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.
+
+// Regression test for issue 11891.
+
+import "package:expect/expect.dart";
+import "mixin_prefix_lib.dart";
+
+class A extends Object with MixinClass {
+  String baz() => bar();
+}
+
+void main() {
+  var a = new A();
+  Expect.equals('{"a":1}', a.baz());
+}