Add @overrides to (a lot of) analyzer.

pkg/analyzer/lib/analyzer.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer;
 
 import 'dart:io';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/dart/scanner/reader.dart';
@@ skipping 109 matching lines @@
 
   _ErrorCollector();
 
   /// The group of errors collected.
   AnalyzerErrorGroup get group =>
       new AnalyzerErrorGroup.fromAnalysisErrors(_errors);
 
   /// Whether any errors where collected.
   bool get hasErrors => !_errors.isEmpty;
 
+  @override
   void onError(AnalysisError error) => _errors.add(error);
 }