[flutter_tools] run web unit tests in sound null safety (#70799)

diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index 8bd0494..2aca003 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -1126,13 +1126,13 @@
         '--concurrency=1',  // do not parallelize on Cirrus, to reduce flakiness
       '-v',
       '--platform=chrome',
+      '--sound-null-safety', // web tests do not autodetect yet.
       ...?flutterTestArgs,
       ...tests,
     ],
     workingDirectory: workingDirectory,
     environment: <String, String>{
       'FLUTTER_WEB': 'true',
-      'FLUTTER_LOW_RESOURCE_MODE': 'true',
     },
   );
 }