Add dependency_overrides for null-safe deps that are not yet published

PiperOrigin-RevId: 335128497
diff --git a/pubspec.yaml b/pubspec.yaml
index f1c6a9c..4c4ba93 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -17,3 +17,9 @@
 dev_dependencies:
   fixnum: '>=0.9.0 <0.11.0'
   test: ^1.16.0-nullsafety.4
+
+dependency_overrides:
+  clock:
+    git: https://github.com/dart-lang/clock
+  fixnum:
+    git: https://github.com/dart-lang/fixnum