Simplify conversion of empty lists to null.

pkg/analysis_server/lib/src/edit/edit_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 edit.domain;
 
 import 'dart:async';
 
+import 'package:analysis_server/src/collections.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol.dart' hide Element;
 import 'package:analysis_server/src/services/correction/assist.dart';
 import 'package:analysis_server/src/services/correction/fix.dart';
 import 'package:analysis_server/src/services/correction/sort_members.dart';
 import 'package:analysis_server/src/services/correction/status.dart';
 import 'package:analysis_server/src/services/refactoring/refactoring.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 254 matching lines @@
       }
       // validation and create change
       refactoring.checkFinalConditions().then((_finalStatus) {
         finalStatus = _finalStatus;
         if (_hasFatalError) {
           return _sendResultResponse();
         }
         // create change
         return refactoring.createChange().then((change) {
           result.change = change;
-          if (!refactoring.potentialEditIds.isEmpty) {
-            result.potentialEdits = refactoring.potentialEditIds;
-          }
+          result.potentialEdits = nullIfEmpty(refactoring.potentialEditIds);
           return _sendResultResponse();
         });
       });
     });
   }
 
   /**
    * Initializes this context to perform a refactoring with the specified
    * parameters. The existing [Refactoring] is reused or created as needed.
    */
@@ skipping 196 matching lines @@
     }
     if (refactoring is RenameRefactoring) {
       RenameRefactoring renameRefactoring = refactoring;
       RenameOptions renameOptions = params.options;
       renameRefactoring.newName = renameOptions.newName;
       return renameRefactoring.checkNewName();
     }
     return new RefactoringStatus();
   }
 }