function createPlusField(args = undefined) {
  const plus = new Blockly.FieldImage(plusImage, 15, 15, undefined, onClick_);
  /**
   * Untyped args passed to block.plus when the field is clicked.
   * @type {?(Object|undefined)}
   * @private
   */
  plus.args_ = args;
  return plus;
}
Creates a plus image field used for mutation.
@param {Object=} args Untyped args passed to block.minus when the field
    is clicked.
@returns {Blockly.FieldImage} The Plus field.