Add conversion routines from the plugin API to the server API

pkg/analysis_server/lib/src/plugin/result_collector.dart

 // Copyright (c) 2017, 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.
 
 /**
  * An object used to collect partial results (of type [E]) where the partial
  * results are contributed by plugins.
  */
 class ResultCollector<E> {
   /**
@@ skipping 48 matching lines @@
     // Ensure that the server's contributions are always first in the list.
     //
     E serverContributions = partialResultMap[serverId];
     if (serverContributions != null && values.remove(serverContributions)) {
       values.insert(0, serverContributions);
     }
     return values;
   }
 
   /**
+   * Return `true` if this collector is collecting results associated with the
+   * given [filePath].
+   */
+  bool isCollectingFor(String filePath) => resultMap.containsKey(filePath);
+
+  /**
    * Record the [partialResults] as having been contributed for the given
    * [filePath] by the plugin with the given [pluginId].
    */
   void putResults(String filePath, String pluginId, E partialResults) {
     Map<String, E> fileResults = resultMap[filePath];
     if (fileResults != null) {
       fileResults[pluginId] = partialResults;
     }
   }
 
   /**
    * Start collecting results contributed for the file with the given
    * [filePath]. Unless the collector is told to collect results for a file, any
    * results that are contributed for that file are discarded.
    */
   void startCollectingFor(String filePath) {
     resultMap.putIfAbsent(filePath, () => <String, E>{});
   }
 
   /**
    * Stop collecting results contributed for the file with the given [filePath].
    * Until the collector is told to start collecting results for the file, any
    * results that are contributed for the file are discarded.
    */
   void stopCollectingFor(String filePath) {
     resultMap.remove(filePath);
   }
 }