Hide outdated --mode flag (#2603)

diff --git a/lib/src/command/outdated.dart b/lib/src/command/outdated.dart
index 85f0e57..f24c317 100644
--- a/lib/src/command/outdated.dart
+++ b/lib/src/command/outdated.dart
@@ -66,7 +66,9 @@
             '--show-all.',
         valueHelp: 'PROPERTY',
         allowed: ['outdated', 'null-safety'],
-        defaultsTo: 'outdated');
+        defaultsTo: 'outdated',
+        hide: true // TODO(sigurdm): Unhide when null-safety is launched.
+        );
 
     argParser.addFlag('prereleases',
         help: 'Include prereleases in latest version.');
diff --git a/test/outdated/goldens/helptext.txt b/test/outdated/goldens/helptext.txt
index 5bd10b4..9b43f78 100644
--- a/test/outdated/goldens/helptext.txt
+++ b/test/outdated/goldens/helptext.txt
@@ -11,10 +11,6 @@
     --[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-                                   [outdated (default), null-safety]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.