Allow null-safe code to do `same(null)`. (#161)

diff --git a/lib/src/core_matchers.dart b/lib/src/core_matchers.dart
index 91061d7..c049109 100644
--- a/lib/src/core_matchers.dart
+++ b/lib/src/core_matchers.dart
@@ -106,7 +106,7 @@
 Matcher same(expected) => _IsSameAs(expected);
 
 class _IsSameAs extends Matcher {
-  final Object _expected;
+  final Object? _expected;
   const _IsSameAs(this._expected);
   @override
   bool matches(item, Map matchState) => identical(item, _expected);