Returns the first index of element in this list.
Searches the list from index start to the end of the list.
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.indexOf('re'); // 1
notes.indexOf('re', 2); // 3
Returns -1 if element is not found.
notes.indexOf('fa'); // -1
Source
int indexOf(E element, [int start = 0]);