Bump foundation 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_FOUNDATION:
- pkg/build_integration
- pkg/scrape

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

Change-Id: Ib38f7648f1d170756e1e18c0177ce5d76a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/487881
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/build_integration/pubspec.yaml

@@ -6,7 +6,7 @@ description: >
 publish_to: none
 
 environment:
-  sdk: ^3.5.0
+  sdk: '^3.12.0-0'
 
 resolution: workspace
 

pkg/scrape/pubspec.yaml

@@ -4,7 +4,7 @@ description: Helper package for analyzing the syntax of Dart programs.
 publish_to: none
 
 environment:
-  sdk: ^3.5.0
+  sdk: '^3.12.0-0'
 
 resolution: workspace