[dart2js] List.insertAll can leave unsound null elements when interrupted

[askeksa-google]

Similarly to #48132, List.insertAll can leave null elements in a list with a non-nullable element type if the operation is interrupted by an exception:

import 'dart:collection';

class BadList extends ListBase<int> {
  int get length => 2;
  int operator [](int index) => throw "BOOM";

  set length(int l) => throw "unimplemented";
  void operator []=(int index, int value) => throw "unimplemented";
  void add(int e) => throw "unimplemented";
}

main() {
  List<int> list = [1, 2];
  try {
    list.insertAll(1, BadList());
  } catch (_) {}
  print(list.length);
  print(list[2]);
}

This prints either

4
null

or

4
undefined

depending on optimization level.

[lrhn]

We should prevent unsound results.
The big question is whether to prioritize correctness or performance when the incoming iterable is broken (because prioritizing correctness will likely hurt performance when everything is correct too, but a completely random result when things break can be arbitrarily bad).

See the similar VM issue (#48132) for a suggestion, which may or may not apply to JS as well.