Add ListExtensions.firstOrNull, lastOrNull, singleOrNull.

[scheglov]

See dart-archive/collection#312

For List we don't have to instantiate Iterator to implement these methods.

[lrhn]

We could also let the existing operation on Iterable check for whether the Iterable has efficient length. If it has, it can do the same length checks as here, also for sets and queues, then use first/last on the Iterable for the value.

The lastOrNull code already does this.

[scheglov]

From performance perspective this seems bad. If we statically know that it is a List, why should we lose this information and invoke a more generic version, that then checks for the type to recover the lost information? Or do we know that VM will inline such methods, and rewrite by leaving only the branch for List types, practically removing the type check?

[lrhn]

I don't think we know that anything will be inlined.
The extension on List will clearly be better, but will not work for Set or Queue, or on a list which is not statically known to be a list.

So we could do both.