code: function marshallCustomIdentifiersHelper(result, params, shape) {
    if (!shape || params === undefined || params === null) return;
    if (shape.type === 'structure' && shape.required && shape.required.length > 0) {
      util.arrayEach(shape.required, function(name) {
        var memberShape = shape.members[name];
        if (memberShape.endpointDiscoveryId === true) {
          var locationName = memberShape.isLocationName ? memberShape.name : name;
          result[locationName] = String(params[name]);
        } else {
          marshallCustomIdentifiersHelper(result, params[name], memberShape);
        }
      });
    }
  }
docs: Recursive helper for marshallCustomIdentifiers().
  Looks for required string input members that have 'endpointdiscoveryid' trait.
  @api private