DeCo. Comment out assert(nameOffset != null ...)

[scheglov]

It is not true anymore (starting from https://dart-review.googlesource.com/c/sdk/+/466441) for extension types: the representation field is not present in code, so should not have any name offset. And its getter / setter are also synthetic, and also don't have any name offset.

So, in this PR I comment out the assert, and leave it to you to implement the semantics that you think is right for DartDoc.