function isEllipsisString( prop ) {
	return ( prop === '...' );
}
Tests if an indexing expression is an ellipsis expression.

@private
@param {string} prop - property name
@returns {boolean} result

@example
var out = isEllipsisString( '...' );
// returns true

@example
var out = isEllipsisString( '::-2' );
// returns false

@example
var out = isEllipsisString( '-2' );
// returns false