Quick pass at fixing a few dangling references as revealed by the new `comment_references` lint (https://github.com/dart-lang/linter/issues/240).

There's a bunch more to do here before we can turn it on by default (~430 lints as of now).  Many of them are a simple matter of adding an import (e.g., `dart:async` for library docs that reference `Future`) but others will require a bit of thought.  Probably best done by the folks writing the code. :)