code: function (newBackingValue) {
      var newSharedInternals = {};
      Util.assign(newSharedInternals, this._sharedInternals);
      newSharedInternals.backingValue = newBackingValue;
      return new Binding(this._path, newSharedInternals);
    }
docs: Update backing value.
  @param {Immutable.Map} newBackingValue new backing value
  @return {Binding} new binding instance, original is unaffected