Support @docImports

[srawlins]

There is much work to do, and this is more of an umbrella ticket.

• Support doc comment references that are resolvable by the new @docImport syntax. Introduce a syntax for importing elements just for doc comment references sdk#50702
• Migrate flutter/flutter and flutter/engine to use this new syntax, and stop using dartdocs 'reference children' universal scope system.

[matanlurey]

Maybe worth mentioning here or a separate bug:

Sometimes @docImport shows up in the generated docs, and I need to comment it out:

/// <!--
/// @docImport 'library.dart';
/// -->
library;

[srawlins]

Ooh that "sometimes" would be good to know.

[matanlurey]

The next time I have to do it, you'll be the first to know :)

Ooh that "sometimes" would be good to know.

Looks like it happened here:

https://github.com/matanlurey/pxl.dart/blob/c379ef6ed22e14307e5fc3af5d1dc833aa226069/lib/pxl.dart#L11

And here:

https://github.com/matanlurey/pub.lurey.dev/blob/b4836f5588ce2525a7a67d2635f02a41aa81b11e/packages/proc/lib/proc.dart#L71