Remove use of private metadata extension (#4053)

Fixes #4052

The awkwardness just comes from how `metadata2` is not declared on
Element2, but on Annotatable. So a little helper extension at the end
helps us keep code concise.

Also:

* bump the CI Dart version to 3.7.0 (could probably bump to 3.8.0 soon
enough 🤷 )
* adjust a test since `@Required` is recently deprecated.
* adjust a test which includes new formatting for 3.7.0, I believe.

Author: srawlins

.github/workflows/test.yaml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        sdk: [3.6.0, dev]
+        sdk: [3.7.0, dev]
         job: [main, flutter, packages, sdk-docs]
         include:
           - os: macos-latest
@@ -35,9 +35,9 @@ jobs:
             # Do not try to run flutter against the "stable" sdk,
             # it is unlikely to work and produces uninteresting
             # results.
-          - sdk: 3.6.0
+          - sdk: 3.7.0
             job: flutter
-          - sdk: 3.6.0
+          - sdk: 3.7.0
             job: sdk-docs
 
     steps: