function Key$1(params) {
    Operator.call(this, null, update$2, params);
  }
Generates a key function.
@constructor
@param {object} params - The parameters for this operator.
@param {Array<string>} params.fields - The field name(s) for the key function.
@param {boolean} params.flat - A boolean flag indicating if the field names
 should be treated as flat property names, side-stepping nested field
 lookups normally indicated by dot or bracket notation.