function isSliceString( prop ) {
	return (
		prop[ 0 ] === 'S' &&
		startsWith( prop, 'Slice(', 0 ) &&
		prop[ prop.length-1 ] === ')'
	);
}
Tests if an indexing expression is a serialized Slice object.

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

@example
var out = isSliceString( 'Slice(0,10,2)' );
// returns true

@example
var out = isSliceString( ':' );
// returns false