ata mutation and path notification API
   * (e.g. `notifyPath`, `set`, and array mutation API's) that allow efficient
   * mutation and notification of deep changes in an object graph to all elements
   * bound to the same object graph.
   *
   * In cases where neither immutable patterns nor the data mutation API can be
   * used, applying this mixin will allow Polymer to skip dirty checking for
   * objects and arrays (always consider them to be "dirty").  This allows a
   * user to make a deep modificatio