Fix two issues with modular compilation in dartk.

tests/language/mixin_class_from_core_library_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 'dart:collection';
+
+class MyList extends Object with ListMixin {
+  int length = 0;
+  operator[](index) => null;
+  void operator[]=(index, value) {}
+}
+
+main() {
+  new MyList().length;
+}