[mraleph]

The operator itself would throw an error.

Dart is planning to have a sound non-nullability which means that if something has static type `SomeType` then it is guaranteed to be a value that is actually `SomeType` and not something else.

Thus you can't take `SomeType?` and just cast it to `SomeType` because `SomeType?` can be null and `SomeType` can't be null.

This means this cast has to perform a check and throw if your value is null.

This also means that if you have `e1` and `e1` has non-nullable static type then `e1` would never be null in the NNBD world - which is a very good property (e.g. for optimizations).

[williamdclt]

Interesting! So you were right to call it an assertion rather than a cast