int indexOf(Object element, [ int startIndex = 0 ])

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(Object element, [int startIndex = 0]) {
  if (startIndex >= this.length) {
    return -1;
  }
  if (startIndex < 0) {
    startIndex = 0;
  }
  for (int i = startIndex; i < this.length; i++) {
    if (this[i] == element) {
      return i;
    }
  }
  return -1;
}