Avoid redundant calls to "pub list" when multiple dependencies change.

pkg/analysis_server/lib/src/source/caching_pub_package_map_provider.dart

 // Copyright (c) 2015, 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 source.caching_pub_package_map_provider;
 
 import 'dart:convert';
 import 'dart:io' as io;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/source/package_map_provider.dart';
 import 'package:analyzer/source/pub_package_map_provider.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk_io.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * The function used to write the cache file.
  * Returns the modification stamp for the newly written file.
  */
 typedef int WriteFile(File file, String content);
 
 /**
  * [PubPackageMapProvider] extension which caches pub list results.
  * These results are cached in memory and in a single place on disk that is
  * shared cross session and between different simultaneous sessions.
+ *
+ * TODO(paulberry): before this class is used again, it should be ported over
+ * to extend OptimizingPubPackageMapProvider instead of PubPackageMapProvider.
  */
 class CachingPubPackageMapProvider extends PubPackageMapProvider {
   static const cacheKey = 'pub_list_cache';
   static const cacheVersion = 1;
   static const cacheVersionKey = 'pub_list_cache_version';
   static const pubListResultKey = 'pub_list_result';
   static const modificationStampsKey = 'modification_stamps';
 
   /**
    * A cache of folder path to pub list information as shown below
@@ skipping 209 matching lines @@
     // TODO(danrubel) This implementation assumes that
     // two separate processes are not accessing the cache file at the same time
     io.File file = new io.File(cacheFile.path);
     if (!file.parent.existsSync()) {
       file.parent.createSync(recursive: true);
     }
     file.writeAsStringSync(content, flush: true);
     return file.lastModifiedSync().millisecondsSinceEpoch;
   }
 }