Support tests with breaking changes in Dart 3.0.0

[elliette]

This issue tracks the work involved to fix all the tests so that they are compatible with both version 3.0.0 and older versions.

Work involved:

• Get all the CI tests to pass to unblock merging in changes
  • Completed in Update tests that are incompatible with 3.0.0 #1817
• Make nullsafety mode a required option in test context set up
  • Completed in Refactor TestContext constructor for clarity #1844
• Re-enable skipped ChromeProxyService test cases
  Note: some tests are failing due to different code in sound fixtures, ie
  Classes (MyTestClass has different fields, update to match)
• Migrate the events_test to null-safety (somehow it was missed in the null-safety migration)
  • Completed in Migrate events_test.dart to null-safety #1819
• Support Frontend Server compilation with sound null-safety
  • Tracked in Support running frontend server tests with sound null safety. #1591
• Support Frontend Server and Build Daemon compilation with weak null-safety
  • Completed in Enable weak tests for build daemon and frontend server #1824
• Run all tests (where applicable) in both weak and sound null-safety modes. Most of our tests were using the weak null-safety test fixtures, which is what caused the failures.
• Re-enable disabled test in reload_test. The callback passed to registerExtension requires a non-null return type [1].
  • Completed in Re-enable skipped reload test #1958
• Re-enable the frontend_server_asset_reader_tests
  • Those are already running...
• dart_uri_file_uri_test can be for sound null-safety only.
  • Completed in Make dart-uri-file-uri test use sound null safety #1959

  [FINEST] ExpressionCompilerService: Failed to compile "() => registerExtension('ext.foo', (method, params) {})": org-dartlang-debug:synthetic_debug_expression:1:36: Error: A non-null value must be returned since the return type 'Future<ServiceExtensionResponse>' doesn't allow null.
   - 'Future' is from 'dart:async'.
   - 'ServiceExtensionResponse' is from 'dart:developer'.
  () => registerExtension('ext.foo', (method, params) {})                               ^