update deprecated window property accesses

[pq]

The Dart SDK bots are currently failing due to deprecated member accesses:

Running "flutter pub get" in gallery...                         
   15.2s
+ /b/s/w/ir/cache/builder/sdk/out/ReleaseX64/dart-sdk/bin/dart analyze --fatal-infos
Analyzing gallery...

   info - test_goldens/testing/util.dart:36:25 - 'textScaleFactorTestValue' is deprecated and shouldn't be used. Use platformDispatcher.textScaleFactorTestValue instead. This feature was deprecated after v2.11.0-0.0.pre.. Try replacing the use of the deprecated member with the replacement. - deprecated_member_use
   info - test_goldens/testing/util.dart:37:25 - 'platformBrightnessTestValue' is deprecated and shouldn't be used. Use platformDispatcher.platformBrightnessTestValue instead. This feature was deprecated after v2.11.0-0.0.pre.. Try replacing the use of the deprecated member with the replacement. - deprecated_member_use

2 issues found.

See: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8820350444085541905/+/u/analyze_flutter_gallery/stdout

This should fix them up.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[dnfield]

This LGTM by itself, but I'm curious why it started failing and if something in analysis changed that we should be aware of more broadly.