findIndex method
Finds the index of the first element that satisfies the given predicate.
Returns the index of the first element in the iterable
that matches the provided test
function.
Returns -1 if no such element is found.
Implementation
int findIndex(ConditionTester<T> test) {
for (var i = 0; i < length; i++) {
if (test(elementAt(i))) {
return i;
}
}
return -1;
}