Make .flatten() type-safe

[passsy]

Removes Iterable<E>.flatten(): Iterable<dynamic> in favor of

• Iterable<Itearble<E>>.flatten(): Iterable<E> and
• List<List<E>>.flatten(): List<E>

The old implementation wasn't type safe and risks a class cast exception

  Iterable<dynamic> flatten() sync* {
    for (var current in this) {
      yield* (current as Iterable); // cast may fail
    }
  }

I consider this change as almost-non-breaking 😁
If flatten() was called on a non-nested Iterable<E> where E is not a Iterable it would fail at runtime. So the code was already broken.
The only exception is when Iterable<dynamic> is empty. For this code the method will not be available anymore.

This PR is a alternative to #55

[shinayser]

DO IT!

[simc]

👍