not be propagated unless "immutable" data patterns are used (i.e. all object
   * references from the root to the mutation were changed).
   *
   * Polymer also provides a proprietary data 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
   * 