Describe the issue
According to #3049, collection_methods_unrelated_type should warn about using null as a key to a non-nullable collection. However, the following code does not generate the lint:

final v = <String, String>{ “somekey” : “somevalue” };
final nv = v[null];

If this is expected, I propose another lint that will warn about indexing into collections with non-null keys with potentially null values.

To Reproduce

The following code:

final v = <String, String>{ “somekey” : “somevalue” };
final nv = v[null];

Expected behavior
Warn on line 2 about null being used to on a map with non-null keys.