[tool] Update to Dart 3 (#6030)

Updates the repo tooling to Dart 3, now that the N-2 version has Dart 3, which allows us to use Dart 3 features (e.g., records) going forward.

To allow the update:
- Removes `break` commands from `switch`es (all done automatically with `dart fix --apply`)
- Replaces mocking of `ProcessResult` with just creating an actual `ProcessResult` since it's a `final` data class and thus can't (but also doesn't need to be) mocked.

script/tool/lib/src/version_check_command.dart

@@ -210,20 +210,16 @@ class VersionCheckCommand extends PackageLoopingCommand {
     switch (versionState) {
       case _CurrentVersionState.unchanged:
         versionChanged = false;
-        break;
       case _CurrentVersionState.validIncrease:
       case _CurrentVersionState.validRevert:
       case _CurrentVersionState.newPackage:
         versionChanged = true;
-        break;
       case _CurrentVersionState.invalidChange:
         versionChanged = true;
         errors.add('Disallowed version change.');
-        break;
       case _CurrentVersionState.unknown:
         versionChanged = false;
         errors.add('Unable to determine previous version.');
-        break;
     }
 
     if (!(await _validateChangelogVersion(package,