Add conversion routines from the plugin API to the server API

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

Index: pkg/analysis_server/lib/src/plugin/result_collector.dart
diff --git a/pkg/analysis_server/lib/src/plugin/result_collector.dart b/pkg/analysis_server/lib/src/plugin/result_collector.dart
index d5491b97e91909489ccfa619d7a5e9dcd55a293b..80c6dc738a5fe254510e5740110d6d50e2754d02 100644
--- a/pkg/analysis_server/lib/src/plugin/result_collector.dart
+++ b/pkg/analysis_server/lib/src/plugin/result_collector.dart
@@ -66,6 +66,12 @@ class ResultCollector<E> {
   }
 
   /**
+   * 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].
    */