Problem: Existing interfaces with generic types may be only partially supportable with nullable types

Concrete example: `List(1);` is safe, but `List(1)` will most likely need to throw at runtime because `[null]` is not a `List` with NNBD. `List.filled(1, 0);` would be safe.

Start with the concrete `List` and `List` examples in the issue and review the language specification areas covering NNBD and generic interface types. Done would require a decided, specified treatment of partially supportable nullable generic interfaces, including the runtime behavior for the examples; no files or tests are named.

Written by the indexing model from the issue text.