Returns the last index of element in this list.
Searches the list backwards from index start to 0.
The first time an object o is encountered so that o == element,
the index of o is returned.
List<String> notes = ['do', 're', 'mi', 're'];
notes.lastIndexOf('re', 2); // 1
If start is not provided, this method searches from the end of the
list./Returns
notes.lastIndexOf('re'); // 3
Returns -1 if element is not found.
notes.lastIndexOf('fa'); // -1
Source
int lastIndexOf(E element, [int start]);