userAppend method
Updates user properties of list type by appending some elements.
Implementation
void userAppend(Map<String, List> properties) {
properties.updateAll((String k, List v) {
return v.map((e) => e is DateTime ? _formatDateString(e) : e).toList();
});
_channel.invokeMethod<void>('userAppend',
<String, dynamic>{'properties': properties, 'appId': this._appId});
}