a === b -> identical(a, b)

pkg/unittest/lib/src/core_matchers.dart

Index: pkg/unittest/lib/src/core_matchers.dart
diff --git a/pkg/unittest/lib/src/core_matchers.dart b/pkg/unittest/lib/src/core_matchers.dart
index 046da2c699b5c7217e0490989738eae7c7dca1af..707b32b2121030bc6689c0e49813db98adc07a00 100644
--- a/pkg/unittest/lib/src/core_matchers.dart
+++ b/pkg/unittest/lib/src/core_matchers.dart
@@ -73,7 +73,7 @@ Matcher same(expected) => new _IsSameAs(expected);
 class _IsSameAs extends BaseMatcher {
   final _expected;
   const _IsSameAs(this._expected);
-  bool matches(item, MatchState matchState) => item === _expected;
+  bool matches(item, MatchState matchState) => identical(item, _expected);
   // If all types were hashable we could show a hash here.
   Description describe(Description description) =>
       description.add('same instance as ').addDescriptionOf(_expected);