function prop2array( property, cache ) {
	var o = cache.get( getIdentifier( trim( property ) ) );
	if ( o === null ) {
		throw new Error( format( 'invalid operation. Unable to resolve ndarray index. Value: `%s`.', property ) );
	}
	return o;
}
Converts an indexing expression to an ndarray index.

@private
@param {string} property - property name
@param {Object} cache - cache for resolving ndarray index objects
@throws {Error} invalid ndarray index
@returns {(Object|null)} index object (or null)