Bump unowned 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 have empty OWNERS
files:
- pkg/heap_snapshot
- pkg/perf_witness
- pkg/shell_arg_splitter

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

Change-Id: Ie5c9dbd116be3537e84b1df4ca93d8d06a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/487920
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>

pkg/heap_snapshot/pubspec.yaml

@@ -3,7 +3,7 @@ name: heap_snapshot
 publish_to: none
 
 environment:
-  sdk: ^3.5.0
+  sdk: '^3.12.0-0'
 
 resolution: workspace
 

pkg/perf_witness/pubspec.yaml

@@ -6,7 +6,7 @@ publish_to: none
 resolution: workspace
 
 environment:
-  sdk: ^3.11.0-0
+  sdk: '^3.12.0-0'
 
 dependencies:
   args: any

pkg/shell_arg_splitter/pubspec.yaml

@@ -3,7 +3,7 @@ name: shell_arg_splitter
 publish_to: none
 
 environment:
-  sdk: '^3.7.0'
+  sdk: '^3.12.0-0'
 
 resolution: workspace