function getWarnings() {
	var msg;
	var out;
	var i;

	out = [];
	for ( i = 0; i < this.messages.length; i++ ) {
		msg = this.messages[ i ];
		if ( msg.severity === 'warn' ) {
			out.push( msg );
		}
	}
	if ( out.length === 0 ) {
		return null;
	}
	return out;
}
Returns array of encountered warning messages.

@private
@returns {(Array|null)} warning messages or `null`