make sourcespan non-nullable

pkgs/yaml/test/span_test.dart

diff --git a/pkgs/yaml/test/span_test.dart b/pkgs/yaml/test/span_test.dart
index 6c666dd..b8170e6 100644
--- a/pkgs/yaml/test/span_test.dart
+++ b/pkgs/yaml/test/span_test.dart
@@ -8,8 +8,8 @@
 import 'package:test/test.dart';
 import 'package:yaml/yaml.dart';
 
-void _expectSpan(SourceSpan? source, String expected) {
-  final result = source!.message('message');
+void _expectSpan(SourceSpan source, String expected) {
+  final result = source.message('message');
   printOnFailure("r'''\n$result'''");
 
   expect(result, expected);