Re-enable --mark=null-safety (#2685)

diff --git a/lib/src/command/outdated.dart b/lib/src/command/outdated.dart
index c29f643..6591238 100644
--- a/lib/src/command/outdated.dart
+++ b/lib/src/command/outdated.dart
@@ -66,9 +66,7 @@
             '--show-all.',
         valueHelp: 'PROPERTY',
         allowed: ['outdated', 'null-safety'],
-        defaultsTo: 'outdated',
-        hide: true // TODO(sigurdm): Unhide when null-safety is launched.
-        );
+        defaultsTo: 'outdated');
 
     argParser.addFlag('prereleases',
         help: 'Include prereleases in latest version.');
diff --git a/test/outdated/goldens/helptext.txt b/test/outdated/goldens/helptext.txt
index 9b43f78..5bd10b4 100644
--- a/test/outdated/goldens/helptext.txt
+++ b/test/outdated/goldens/helptext.txt
@@ -11,6 +11,10 @@
     --[no-]dev-dependencies        Take dev dependencies into account.
                                    (defaults to on)
     --json                         Output the results using a json format.
+    --mode=<PROPERTY>              Highlight versions with PROPERTY.
+                                   Only packages currently missing that PROPERTY
+                                   will be included unless --show-all.
+                                   [outdated (default), null-safety]
     --[no-]prereleases             Include prereleases in latest version.
     --[no-]show-all                Include dependencies that are already
                                    fullfilling --mode.