Remove a sketchy override of a field (#6009)

The way this was implemented is sketchy, especially in strong mode.

packages/flutter_tools/lib/src/commands/test.dart

@@ -36,16 +36,7 @@ class TestCommand extends FlutterCommand {
       defaultsTo: 'coverage/lcov.info',
       help: 'Where to store coverage information (if coverage is enabled).'
     );
-  }
-
-  @override
-  String get name => 'test';
-
-  @override
-  String get description => 'Run Flutter unit tests for the current project.';
-
-  @override
-  Validator commandValidator = () {
+    commandValidator = () {
       if (!FileSystemEntity.isFileSync('pubspec.yaml')) {
         printError(
           'Error: No pubspec.yaml file found in the current working directory.\n'
@@ -56,6 +47,13 @@ class TestCommand extends FlutterCommand {
       }
       return true;
     };
+  }
+
+  @override
+  String get name => 'test';
+
+  @override
+  String get description => 'Run Flutter unit tests for the current project.';
 
   Iterable<String> _findTests(Directory directory) {
     return directory.listSync(recursive: true, followLinks: false)