Add expectations for emitted warnings (#2366)

Pulled from #2362 and refactored to test for exactly 1 warning.

Add a test utility to check for warnings emitted while parsing and use
it in two test cases that emit warnings.

Author: natebosch

pkgs/yaml/CHANGELOG.md

@@ -1,3 +1,5 @@
+## 3.1.4-wip
+
 ## 3.1.3
 
 * Require Dart 3.4

pkgs/yaml/pubspec.yaml

@@ -1,5 +1,5 @@
 name: yaml
-version: 3.1.3
+version: 3.1.4-wip
 description: A parser for YAML, a human-friendly data serialization standard
 repository: https://github.com/dart-lang/tools/tree/main/pkgs/yaml
 issue_tracker: https://github.com/dart-lang/tools/labels/package%3Ayaml

pkgs/yaml/test/utils.dart

@@ -53,6 +53,23 @@ void expectYamlFails(String source) {
   expect(() => loadYaml(cleanUpLiteral(source)), throwsYamlException);
 }
 
+/// Asserts that a string containing a single YAML document produces a given
+/// value and emits warnings when loaded.
+void expectYamlLoadsWithWarning(
+    Object? expected, String source, String expectedWarning) {
+  final warnings = <String>[];
+  final oldCallback = yamlWarningCallback;
+  yamlWarningCallback = (message, [_]) {
+    warnings.add(message);
+  };
+  try {
+    expectYamlLoads(expected, source);
+    expect(warnings, ['Warning: $expectedWarning']);
+  } finally {
+    yamlWarningCallback = oldCallback;
+  }
+}
+
 /// Removes eight spaces of leading indentation from a multiline string.
 ///
 /// Note that this is very sensitive to how the literals are styled. They should

pkgs/yaml/test/yaml_test.dart

@@ -992,21 +992,25 @@ void main() {
   });
 
   group('6.8: Directives', () {
-    // TODO(nweiz): assert that this produces a warning
     test('[Example 6.13]', () {
-      expectYamlLoads('foo', '''
+      expectYamlLoadsWithWarning(
+          'foo',
+          '''
         %FOO  bar baz # Should be ignored
                       # with a warning.
-        --- "foo"''');
+        --- "foo"''',
+          'unknown directive.');
     });
 
-    // TODO(nweiz): assert that this produces a warning.
     test('[Example 6.14]', () {
-      expectYamlLoads('foo', '''
+      expectYamlLoadsWithWarning(
+          'foo',
+          '''
         %YAML 1.3 # Attempt parsing
                    # with a warning
         ---
-        "foo"''');
+        "foo"''',
+          'this parser only supports YAML 1.1 and 1.2.');
     });
 
     test('[Example 6.15]', () {