Improve import prefix search results UI.

pkg/analysis_server/lib/src/search/search_domain.dart

 // Copyright (c) 2014, 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 search.domain;
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analysis_server/src/search/element_references.dart';
@@ skipping 38 matching lines @@
       if (element is ImportElement) {
         return element.prefix;
       }
       if (element is FieldFormalParameterElement) {
         return element.field;
       }
       if (element is PropertyAccessorElement) {
         return element.variable;
       }
       return element;
+    }).where((Element element) {
+      return element != null;
     }).toList();
     // schedule search
     String searchId = (_nextSearchId++).toString();
     elements.forEach((Element element) {
       var computer = new ElementReferencesComputer(searchEngine);
       var future = computer.compute(element, params.includePotential);
       future.then((List<protocol.SearchResult> results) {
         bool isLast = identical(element, elements.last);
         _sendSearchNotification(searchId, isLast, results);
       });
@@ skipping 109 matching lines @@
         new protocol.SearchResultsParams(
             searchId,
             results.toList(),
             isLast).toNotification());
   }
 
   static protocol.SearchResult toResult(SearchMatch match) {
     return protocol.newSearchResult_fromMatch(match);
   }
 }