Work towards enabling the comment_references lint

The comment_references lint is currently disabled in analysis_options_common.yaml:

# - comment_references # blocked on https://github.com/dart-lang/linter/issues/1142

That reference is stale: linter#1142 is closed.

The comment_references lint reports doc comment references that the analyzer cannot resolve in the lexical scope of the file they appear in.

Most of these references still render as working links today, because dartdoc falls back to a universal scope — a global lookup of every element it has loaded — when lexical resolution fails. That fallback is what dartdoc wants to remove (dart-lang/dartdoc#4283). This means that references relying on it will eventually stop resolving. Enabling the lint is how we find them before that happens.

I am fixing the existing reports in separate PRs, grouped by the kind of change involved.

Material and Cupertino are frozen in this repository and their code has been copied to flutter/packages (#188444), so those files are excluded here. The same work will be needed in material_ui and cupertino_ui.

Enabling the lint is a longer-term goal. At least one category cannot be fixed immediately: a library referred to by its bare name, such as [widgets], cannot be expressed with @docImport as it currently works (dart-lang/sdk#56528).

The individual PRs are worth landing regardless — each makes references resolve in the IDE as well as in the generated documentation — so they don't depend on when the lint is finally enabled.

PRs so far: