dartdoc should have a "sdk mode"

[jcollins-g]

There are a few tweaks and post-processing bits for Flutter, mostly hiding references to "package", that could be fixed if dartdoc could either recognize or be told that it is documenting an entire SDK rather than an ordinary package. This is mostly of interest to things besides Flutter that might extend Dart in a similar way, but would also make generating docs a little cleaner for Flutter.