Fix an omit-type lint that is breaking the tree (flutter#172776)

flutter#169445 was stale (not re-based),
causing a tree failure.

Let's just land this versus reverting.

Author: matanlurey

packages/flutter_tools/test/web.shard/chrome_test.dart

@@ -933,7 +933,7 @@ void main() {
   );
 
   testWithoutContext('respects custom user data directory flag', () async {
-    const String customUserDataDir = '/custom/chrome/data/dir';
+    const customUserDataDir = '/custom/chrome/data/dir';
     processManager.addCommand(
       const FakeCommand(
         command: <String>[