Use newer function type syntax for parameters.

[munificent]

When Dartdoc generates documentation for a function with a parameter whose type is a function type (i.e. a callback), the generated docs use the old pre-Dart-2.0 function-typed parameter syntax. Dartdoc does this even if the function itself uses the newer syntax.

For over five years, we have had a recommended lint telling users not to use the old function-typed parameter syntax. Newer Dart users may not be familiar with it at all. Our generated docs should probably use the same syntax that we recommend that users write. :)

Also, I am working on a proposal to drop support for the old function-typed parameter syntax completely. If that happens, then we should definitely stop using the old non-existent syntax. (See dart-lang/language#4782).

This obviously affects the output of Dartdoc pretty significantly, so I'm not sure what landing this entails, but I figured I'd put together a PR and send it out to start that discussion. If now is not the right time to make a change like this, that's OK too, but I do think we should make this change at some point.

Fix #3671.

[gemini-code-assist]

Code Review

This pull request updates the parameter renderer to use the newer function type syntax for parameters whose type is a function, replacing the older "function-typed parameter" syntax. The rendering logic in lib/src/render/parameter_renderer.dart has been simplified, and corresponding tests in test/end2end/model_test.dart and test/parameters_test.dart have been updated and expanded to verify the new output format. There are no review comments to evaluate, so I have no feedback to provide.

[srawlins]

Noice!