move dartdoc into the sdk repo

[devoncarew]

We want to move dartdoc into the sdk repo to better participant in refactorings to the analyzer codebase.

[kevmoo]

Not a bad idea – we have tricks to even keep the history here, me thinks.

[srawlins]

I'd like to resurrect this (it never really died 😄 ). There are some costs we'd like to avoid:

• synchronizing breaking changes from the analyzer
• synchronizing releases of dartdoc into the Dart SDK
• synchronizing releases of dartdoc into google3
• a second issue tracker for the analyzer team (could be alleviated independently by just moving the issue tracker and leaving the code)

some costs of moving to dart-lang/sdk:

• likely much slower CI cycles :(
• a little bit less friendly to external developers
• not-as-varied platform trybots? Maybe not, but need to ensure there are Mac and Windows trybots

[jcollins-g]

@srawlins I put some information about my thoughts on this here: https://docs.google.com/document/d/1wx5SiVfeWsoVCBYco1FVMo_EXWPsqnxCCsKBHNJGTYI/edit?resourcekey=0-KdbEwxHW0kEtr7oAur8ReA#heading=h.ojs0ha5yfw0

[whesse]

This can be done the same way that we migrated the linter repo into the Dart SDK repo. All the steps in that migration need to be followed here. I can add a checklist with those steps to this issue, after reviewing them.

The linter migration was tracked in dart-lang/sdk#59165