Migrate `AnalysisError` to `Diagnostic` (#4596)

lib/src/dart.dart

diff --git a/lib/src/dart.dart b/lib/src/dart.dart
index c79d8b7..07d50eb 100644
--- a/lib/src/dart.dart
+++ b/lib/src/dart.dart
@@ -12,7 +12,7 @@
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/analysis/session.dart';
 import 'package:analyzer/dart/ast/ast.dart';
-import 'package:analyzer/error/error.dart';
+import 'package:analyzer/diagnostic/diagnostic.dart';
 import 'package:frontend_server_client/frontend_server_client.dart';
 import 'package:path/path.dart' as p;
 
@@ -71,9 +71,9 @@
   }
 }
 
-/// An error class that contains multiple [AnalysisError]s.
+/// An error class that contains multiple [Diagnostic]s.
 class AnalyzerErrorGroup implements Exception {
-  final List<AnalysisError> errors;
+  final List<Diagnostic> errors;
 
   AnalyzerErrorGroup(this.errors);