Bump model packages to language version 3.12.

This CL is part of an effort to bump the SDK requirement to `3.12.0-0`
for all the packages in `pkg` that are not published to `pub`, so that
we can get better testing of the "private named parameters" feature.

(Packages that *are* published to `pub` can't be safely bumped yet,
because SDK 3.12 hasn't been released, and I don't want to block those
packages' ability to publish useful updates to customers.)

This change covers the following packages, which are owned by
OWNERS_MODEL:
- pkg/analyzer_cli
- pkg/frontend_server
- pkg/testing

Changes to `pubspec.yaml` files were made manually.

Changes to `.dart` files were made automatically, using `dart fix` to
fix failures in the unnecessary_underscores lint.

Change-Id: Ie444a97b2aed3f9eabfde4963c19cfb26a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/487883
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>

pkg/testing/lib/src/zone_helper.dart

@@ -21,7 +21,7 @@ Future runGuarded(
 }) {
   PrintHandler? printWrapper;
   if (printLineOnStdout != null) {
-    printWrapper = (_, __, ___, String line) {
+    printWrapper = (_, _, _, String line) {
       printLineOnStdout(line);
     };
   }

pkg/testing/pubspec.yaml

@@ -5,7 +5,7 @@ name: testing
 publish_to: none
 
 environment:
-  sdk: ^3.5.0
+  sdk: '^3.12.0-0'
 
 resolution: workspace