Add proposal to drop support for the old function syntax - #4782

We have had a core lint for over five years encouraging users to move
away from the old function typedef syntax to the new generalized
typedef syntax. We should probably take our own advice.

This is particularly relevant to:

dart-lang/language#4782

But even if we don't remove support for the old syntax, our own code
should still be idiomatic.

Also enabled the lint in the packages where I could. This CL does not
fix uses of the old typedefs in generated web libraries. I'll fix the
generator and send out a separate CL for that after this lands:

https://dart-review.googlesource.com/c/sdk/+/557020

TEST=Purely mechanical syntax change
CoreLibraryReviewExempt: Purely mechanical syntax change
Change-Id: I94fa1ec73f0ea5b6c8a0dece4f903b12852052d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/556942
Auto-Submit: Bob Nystrom <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Bob Nystrom <[email protected]>
Reviewed-by: Lasse Nielsen <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

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.

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 dart-lang#3671.