Update clock dependency to null-safe version

PiperOrigin-RevId: 337940704
diff --git a/pubspec.yaml b/pubspec.yaml
index 4c4ba93..dd624f8 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -11,7 +11,7 @@
   sdk: '>=2.11.0-180.0.dev <2.11.0'
 
 dependencies:
-  clock: ^1.0.1
+  clock: ^1.1.0-nullsafety.1
   path: ^1.8.0-nullsafety.1
 
 dev_dependencies:
@@ -19,7 +19,5 @@
   test: ^1.16.0-nullsafety.4
 
 dependency_overrides:
-  clock:
-    git: https://github.com/dart-lang/clock
   fixnum:
     git: https://github.com/dart-lang/fixnum