Extract object creation methods for use by a future test class

pkg/analysis_server/lib/src/plugin/result_merger.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.
 
 import 'dart:collection';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     hide AnalysisErrorFixes;
 import 'package:analyzer_plugin/protocol/protocol_generated.dart' as plugin;
 import 'package:meta/meta.dart';
@@ skipping 469 matching lines @@
     /**
      * Perform a depth first traversal of the outline structure rooted at the
      * given [outline] item, re-building each item if any of its children have
      * been updated by the merge process.
      */
     Outline traverse(Outline outline) {
       Outline copiedOutline = copyMap[outline];
       bool isCopied = copiedOutline != null;
       copiedOutline ??= outline;
       List<Outline> currentChildren = copiedOutline.children;
-      if (currentChildren.isEmpty) {
+      if (currentChildren == null || currentChildren.isEmpty) {
         return outline;
       }
       List<Outline> updatedChildren =
           currentChildren.map((Outline child) => traverse(child)).toList();
       if (currentChildren != updatedChildren) {
         if (!isCopied) {
           return new Outline(
               copiedOutline.element, copiedOutline.offset, copiedOutline.length,
               children: updatedChildren);
         }
@@ skipping 317 matching lines @@
     if (leftEnd < rightStart || leftStart > rightEnd) {
       return false;
     }
     if (!allowNesting) {
       return true;
     }
     return !((leftStart <= rightStart && rightEnd <= leftEnd) ||
         (rightStart <= leftStart && leftEnd <= rightEnd));
   }
 }