[flutter_tools] null assertions off by default for web (#64186)

Like Android/iOS, only enable --null-assertions if asked. Previously this was enabled by default for web, but in general this has proved to be too breaking to enable by default.

#61042
diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index 8d8a04d..f29c253 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -866,7 +866,8 @@
         ...<String>[
           '--enable-experiment',
           'non-nullable',
-          '--no-sound-null-safety'
+          '--no-sound-null-safety',
+          '--null-assertions',
         ],
       '-d',
       'chrome',