Don't crash when tree shaking is disabled

tests/compiler/dart2js/serialization/test_data.dart

Index: tests/compiler/dart2js/serialization/test_data.dart
diff --git a/tests/compiler/dart2js/serialization/test_data.dart b/tests/compiler/dart2js/serialization/test_data.dart
index 326d1fa69380fc0af9424362d5647a6c673cd0ee..87a580f03986db710b367a719b9e87c3dc50517f 100644
--- a/tests/compiler/dart2js/serialization/test_data.dart
+++ b/tests/compiler/dart2js/serialization/test_data.dart
@@ -375,6 +375,16 @@ void foo() {}
 main() => const String.fromEnvironment("foo");
 ''',
   }),
+
+  const Test('Disable tree shaking through reflection', const {
+    'main.dart': '''
+import 'dart:mirrors';
+
+main() {
+  reflect(null).invoke(#toString, []).reflectee;
+}
+''',
+  }, expectedWarningCount: 1),
 ];
 
 class Test {