startAt method
Create a query constrained to only return child nodes with a value greater than or equal to the given value, using the given orderBy directive or priority as default, and optionally only child nodes with a key greater than or equal to the given key.
Implementation
Query startAt(dynamic value, {String key}) {
assert(!_parameters.containsKey('startAt'));
assert(value is String ||
value is bool ||
value is double ||
value is int ||
value == null);
final Map<String, dynamic> parameters = <String, dynamic>{'startAt': value};
if (key != null) parameters['startAtKey'] = key;
return _copyWithParameters(parameters);
}