Not all links get cross-referenced

[Hixie]

For example, on this page:
https://docs.flutter.io/flutter/flutter_test/TestGesture-class.html
...WidgetTester.startGesture doesn't get cross-linked to anything.

• We should cross link anything that's valid.
• We should hard fail any time there's something we're not able to cross-link.

[jcollins-g]

This bug turns out to have hidden some major problems with canonicalization because there were no hard failures (and the warnings were less than useful). So canonicalization increased substantially the number of things that were mislinked or not linked at all in complex packages.

[jcollins-g]

Adding to early adopter milestone because just getting us back to parity from before the attempts to fix other issues on this milestone requires addressing this.

[jcollins-g]

#1390 is a start addressing this issue. We don't yet hard fail, but a future version of dartdoc will have the option to set this as a hard-fail condition.

[jcollins-g]

We are now much better than we used to be at solving this, and cross-links now work in the vast majority of cases (including the referenced example).

What seems to be left here is the option to hard-fail.

[jcollins-g]

The hard fail option is finally here in #1891, and should be fine-grained enough to roll out in Flutter.