indexOf method
override
    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
Implementation
int indexOf(Object element, [int start = 0]) {
  if (start < 0) start = 0;
  for (int i = start; i < this.length; i++) {
    if (this[i] == element) return i;
  }
  return -1;
}