Enable NNBD experiment for dartdoc

[jcollins-g]

This will document any NNBD-enabled packages with "null safety" by default, rather than pretending they aren't null safe.

[coveralls]

Coverage increased (+0.4%) to 92.062% when pulling 4d8d076 on nnbd-experiment-on into 8f5f30e on master.

[srawlins]

This will document any NNBD-enabled packages with "null safety" by default, rather than pretending they aren't null safe.

Does it document any NNBD-not-enabled packages as "null safety" because it still requires opting in (via SDK constraint I guess)?

[jcollins-g]

This will document any NNBD-enabled packages with "null safety" by default, rather than pretending they aren't null safe.

Does it document any NNBD-not-enabled packages as "null safety" because it still requires opting in (via SDK constraint I guess)?

I'm not sure I understand, but if you are asking if we force-override and try to parse a non-migrated package to be null safe, the answer is no. With or without the flag in dartdoc set, we always trust the analyzer to know whether a package is null safe. This only changes what dartdoc will generate for the output for null safe packages.