Update json_serializable to support the latest pkg:analyzer (#1535)

And bump everything to rely on Dart 3.9 – since it's required

Ran ` _PUB_TEST_SDK_VERSION=3.9.0 dart pub downgrade --tighten` to
tighten lower bounds on packages that cannot be solved

Release 6.11.2. (#1534)

Allow build 4.0.0. (#1525)

Release 6.11.0. (#1520)

Release 6.10.0. (#1511)

Migrate to element2 APIs (#1504)

* Prepare to publish 6.10.0-dev.

checked_yaml: prepare release (#1500)

Did some cleanup of the implementation so the control flow reads easier
Added a missing test to handle the `null` case.

Fix for latest analyzer (#1487)

Fixes #1485

Fix extra line with Dart 3.7 syntax (#1478)

Since this package is using 3.6, you can't see the result here
Verified with a one-off in this package and another package to make
sure nothing breaks

Error out if the language version of the target package is too low (#…

…1474)

Much be at least 3.0 since we generate code with case statements

Fixes #1462