[ci] Enable LUCI Dart unit tests (#4378)

Enables the new LUCI Dart unit tests.

There is no corresponding Cirrus removal because that test was
accidentally removed early in
https://github.com/flutter/packages/pull/4352.

Part of https://github.com/flutter/flutter/issues/114373

script/tool/test/dart_test_command_test.dart

@@ -231,6 +231,24 @@ void main() {
           ]));
     });
 
+    test('converts --platform=vm to no argument for flutter test', () async {
+      final RepositoryPackage plugin = createFakePlugin(
+        'some_plugin',
+        packagesDir,
+        extraFiles: <String>['test/empty_test.dart'],
+      );
+
+      await runCapturingPrint(runner, <String>['dart-test', '--platform=vm']);
+
+      expect(
+        processRunner.recordedCalls,
+        orderedEquals(<ProcessCall>[
+          ProcessCall(getFlutterCommand(mockPlatform),
+              const <String>['test', '--color'], plugin.path),
+        ]),
+      );
+    });
+
     test('runs in Chrome when requested for Flutter package', () async {
       final RepositoryPackage package = createFakePackage(
         'a_package',