function Expression$1(params) {
    Operator.call(this, null, update$4, params);
    this.modified(true);
  }
Wraps an expression function with access to external parameters.
@constructor
@param {object} params - The parameters for this operator.
@param {function} params.expr - The expression function. The
 function should accept both a datum and a parameter object.
 This operator's value will be a new function that wraps the
 expression function with access to this operator's parameters.