Revert "Revert "Fix syntax and semantic error detection in yaml_edit. (#2284)"" (#2299)

Author: jonasfj

pkgs/yaml_edit/lib/src/editor.dart

@@ -560,30 +560,41 @@ class YamlEditor {
   /// tree is equal to our expectations by deep equality of values. Throws an
   /// [AssertionError] if the two trees do not match.
   void _performEdit(
-      SourceEdit edit, Iterable<Object?> path, YamlNode expectedNode) {
+    SourceEdit edit,
+    Iterable<Object?> path,
+    YamlNode expectedNode,
+  ) {
     final expectedTree = _deepModify(_contents, path, [], expectedNode);
     final initialYaml = _yaml;
-    _yaml = edit.apply(_yaml);
+    final updatedYaml = edit.apply(_yaml);
 
+    // Check that the edit does actually parse
+    final YamlNode actualTree;
     try {
-      _initialize();
+      actualTree = withYamlWarningCallback(() => loadYamlNode(updatedYaml));
     } on YamlException {
       throw createAssertionError(
-          'Failed to produce valid YAML after modification.',
-          initialYaml,
-          _yaml);
+        'Failed to produce valid YAML after modification.',
+        initialYaml,
+        updatedYaml,
+      );
     }
 
-    final actualTree = withYamlWarningCallback(() => loadYamlNode(_yaml));
+    // Check that the edit is semantically correct
     if (!deepEquals(actualTree, expectedTree)) {
       throw createAssertionError(
-          'Modification did not result in expected result.',
-          initialYaml,
-          _yaml);
+        'Modification did not result in expected result.',
+        initialYaml,
+        updatedYaml,
+      );
     }
 
+    // Update state of YamlEditor, when we've validated that the edit was
+    // semantically correct!
+    _yaml = updatedYaml;
     _contents = actualTree;
     _edits.add(edit);
+    _initialize(); // update tracking of aliases
   }
 
   /// Utility method to produce an updated YAML tree equivalent to converting