Always set the previous token in AstCloner.

pkg/analyzer/lib/src/dart/ast/utilities.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 analyzer.src.dart.ast.utilities;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 24 matching lines @@
   final Map<Token, Token> _clonedTokens = new Map<Token, Token>.identity();
 
   /**
    * The next original token to clone.
    */
   Token _nextToClone;
 
   /**
    * The last cloned token.
    */
-  Token _lastCloned;
+  Token _lastCloned = new Token(TokenType.EOF, -1);

[Brian Wilkerson, 2016/02/18 21:00:34]

The EOF token should be it's own 'previous' token.

 
   /**
    * The offset of the last cloned token.
    */
   int _lastClonedOffset = -1;
 
   /**
    * Initialize a newly created AST cloner to optionally clone tokens while
    * cloning AST nodes if [cloneTokens] is `true`.
    *
@@ skipping 889 matching lines @@
               c2 is DocumentationCommentToken) {
             for (int i = 0; i < c1.references.length; i++) {
               _clonedTokens[c1.references[i]] = c2.references[i];
             }
           }
           c1 = c1.next;
           c2 = c2.next;
         }
       }
       _clonedTokens[token] = clone;
-      _lastCloned?.setNext(clone);
+      _lastCloned.setNext(clone);
       _lastCloned = clone;
       if (token.type == TokenType.EOF) {
         break;
       }
       if (token.offset > stopAfter) {
         _nextToClone = token.next;
         _lastClonedOffset = token.offset;
         break;
       }
       token = token.next;
@@ skipping 5377 matching lines @@
    * Safely visit the given [token], printing the [suffix] after the token if it
    * is non-`null`.
    */
   void _visitTokenWithSuffix(Token token, String suffix) {
     if (token != null) {
       _writer.print(token.lexeme);
       _writer.print(suffix);
     }
   }
 }