Assert loader prints directive warnings in tests

Author: kekavc24

pkgs/yaml/test/yaml_test.dart

@@ -993,21 +993,33 @@ void main() {
   });
 
   group('6.8: Directives', () {
-    // TODO(nweiz): assert that this produces a warning
     test('[Example 6.13]', () {
-      expectYamlLoads('foo', '''
+      expect(
+        () => expectYamlLoads(
+          'foo',
+          '''
         %FOO  bar baz # Should be ignored
                       # with a warning.
-        --- "foo"''');
+        --- "foo"''',
+        ),
+        prints(contains('Warning: unknown directive')),
+      );
     });
 
-    // TODO(nweiz): assert that this produces a warning.
     test('[Example 6.14]', () {
-      expectYamlLoads('foo', '''
+      expect(
+        () => expectYamlLoads(
+          'foo',
+          '''
         %YAML 1.3 # Attempt parsing
                    # with a warning
         ---
-        "foo"''');
+        "foo"''',
+        ),
+        prints(
+          contains('Warning: this parser only supports YAML 1.1 and 1.2.'),
+        ),
+      );
     });
 
     test('[Example 6.15]', () {