code: function saveBoxed(onDoneCallback = UNDEFINED) {
      const boxed = this.boxed;
      if (boxed) {
          const modified = boxed[boxedImmutable.BOXED_GET_THIS].valueOfModified();
          if (modified !== UNDEFINED) {
              this.boxed = UNDEFINED;
              if (this.saveState) {
                  return this.saveState(modified, boxed, onDoneCallback);
              } else {
                  throw new TypeError("Save State Not Supported on this instance, saveState callback not provided");
              }
          }
      } else if (onDoneCallback) {
          onDoneCallback();
      }
  }
docs: save modified state
  @param onDoneCallback    function to call on save complete
  @return {*}