function getBuiltinPropertyNames(source, propertyNames) {
  const regex = /attributes\.(\w+)/g;
  let matches = regex.exec(source);
  while (matches !== null) {
    const name = matches[1];
    // Add the property name if it isn't already in the array.
    if (propertyNames.indexOf(name) === -1) {
      propertyNames.push(name);
    }
    matches = regex.exec(source);
  }
}
Gets all the built-in property names used by the given style
function.

@param {Function} source The style function.
@param {string[]} propertyNames The array of property names to add to.

@private