NNBD: Map [] operator accepts null even when keys are non-nullable

[rrousselGit]

Consider the following:

void main() {
  var map = <String, String>{};

  map[null];
}

The current behavior is, map[null] compiles (and return null)

I would expect it to emit a compilation error instead.

---

This appears to be voluntary, considering ma[null] = correctly does not compile.

Maybe we should have a lint?

[lrhn]

The type of operator[] is V Function(Object?). Just like containsKey it accepts any key as query because it allows the object to be cast to Map<Object?, Object?> and still be useful.

We have lints for Iterable.contains, List.remove and operator==. I'm sure we could add lints for all other similarly permissive methods.

[srawlins]

This is implemented in collection_methods_unrelated_type.