[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/test/util.dart

@@ -306,13 +306,11 @@ String _pluginPlatformSection(
         if (support.hasDartCode) {
           lines.add('        dartPluginClass: FakeDartPlugin');
         }
-        break;
       case platformWeb:
         lines.addAll(<String>[
           '        pluginClass: FakePlugin',
           '        fileName: ${packageName}_web.dart',
         ]);
-        break;
       default:
         assert(false, 'Unrecognized platform: $platform');
         break;