Canonicalize elements in remote packages

[sigurdm]

Attempt at fixing #4075, flutter/flutter#170868

Before dartdoc would canonicalize Future to dart:async when generating sdk docs, but not when generating a link from package docs.

[srawlins]

Nice! Impl looks good to me.

[sigurdm]

Sorry for the long delay on this one.

The tests seem to fail on windows only. Not sure what is going on.

@srawlins do you understand why we get this? Is it because the file system on windows is not case-sensitive?
https://github.com/dart-lang/dartdoc/actions/runs/18772999367/job/53561163122?pr=4076#step:10:673

[srawlins]

Very strange, the PR is so simple, and the affected test is dartdoc_test.dart, I believe, which is looking at the testing/ packages. My guess is the backslashes: in HTML all of the links should be forward slashes. Maybe there is logic somewhere in there that is using filepaths, with backslashes, in a way that was not used before this PR.

[srawlins]

Ah this code suspiciously looks at slashes and has the word remote 🤣

dartdoc/lib/src/model/package.dart

Lines 194 to 196 in 6d5e133

	late final String baseHref = documentedWhere == DocumentLocation.remote
	? (_remoteBaseHref.endsWith('/') ? _remoteBaseHref : '$_remoteBaseHref/')
	: (config.useBaseHref ? '' : htmlBasePlaceholder);

ModelElement.href uses this getter, and ModelElement.href is how each element is linked to.