Remove late modifier for Dart generated context classes

[idraper]

When antlr generates code using the Dart runtime, it creates context classes that use the late modifier on labeled grammar elements. For example:

term : EOF_=EOF?;

would generate a term context class where EOF_ would be declared as:

late Token? EOF_;

However, in the above example, there is the possibility that EOF_ would not be set. This is an error in Dart at runtime when trying to access the EOF_ member since the late keyword means Dart expects the value to be assigned before use. The late modifier should be removed since the type is already nullable and thus is not required to be assigned (see here for details about null safety with late).

Thus, the runtime should just generate:

Token? EOF_;

[lingyv-li]

Thanks for your contribution. Can you please provide a reproducible example of when this would fail?

@renancaraujo @canastro can you please take a look at this PR? Thanks

[canastro]

The logic seems sound and the code seems good to go. But, it would be nice to have someone from the core to confirm that EOF_ can actually be not set and its not just some other bug creeping in. I don't have enough knowledge on antlr to confirm or not.

[parrt]

@lingyv-li if you think it looks correct and useful, we can merge.

[lingyv-li]

@parrt Yes this LGTM