Address null safety exclusions in flutter/gallery

[devoncarew]

See flutter/flutter#110016 for context.

test_benchmarks/benchmarks/client.dart:1:// @dart=2.9
test_benchmarks/benchmarks/common.dart:1:// @dart=2.9
test_benchmarks/benchmarks/gallery_automator.dart:5:// @dart=2.9
test_benchmarks/benchmarks/gallery_recorder.dart:5:// @dart=2.9
test_benchmarks/benchmarks/project_root_directory.dart:1:// @dart=2.9
test_benchmarks/benchmarks/run_benchmarks.dart:1:// @dart=2.9
test_benchmarks/benchmarks/scroll.dart:5:// @dart=2.9
test_benchmarks/benchmarks_test.dart:1:// @dart=2.9
test_benchmarks/web_bundle_size_test.dart:5:// @dart=2.9

[mit-mit]

Hey @guidezpl would it be possible to get this migrated to null safety (i.e. remove the @dart=2.9 language markers)?

[guidezpl]

@pennzht could you take a look?

[amanv8060]

this was blocked on package migration ( #527 (comment)). PR for package migration has been merged and new version is available (https://pub.dev/packages/web_benchmarks).
PR for migrating test_benchmarks : #757

[pennzht]

Thank you for your message. Taking a look now.