method
checkValidIndex

void checkValidIndex(
int index,
indexable,
[String name,
int length,
String message]
)

Check that a value is a valid index into an indexable object.

Throws if index is not a valid index into indexable.

An indexable object is one that has a length and a and index-operator [] that accepts an index if 0 <= index < length.

If length is provided, it is used as the length of the indexable object, otherwise the length is found as indexable.length.