Always set the previous token in AstCloner.

pkg/analyzer/test/generated/utilities_test.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.test.generated.utilities_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 1176 matching lines @@
   static void _assertEqualToken(Token clone, Token original) {
     expect(clone.type, original.type);
     expect(clone.offset, original.offset);
     expect(clone.length, original.length);
     expect(clone.lexeme, original.lexeme);
   }
 
   static void _assertEqualTokens(AstNode cloneNode, AstNode originalNode) {
     Token clone = cloneNode.beginToken;
     Token original = originalNode.beginToken;
+    if (original is! CommentToken) {
+      expect(original.previous, isNotNull);
+      expect(clone.previous, isNotNull);
+    }
     Token stopOriginalToken = originalNode.endToken.next;
     Token skipCloneComment = null;
     Token skipOriginalComment = null;
     while (original != stopOriginalToken) {
       expect(clone, isNotNull);
       _assertEqualToken(clone, original);
       // comments
       {
         Token cloneComment = clone.precedingComments;
         Token originalComment = original.precedingComments;
@@ skipping 3100 matching lines @@
   }
 
   void test_get_added() {
     TokenMap tokenMap = new TokenMap();
     Token key = TokenFactory.tokenFromType(TokenType.AT);
     Token value = TokenFactory.tokenFromType(TokenType.AT);
     tokenMap.put(key, value);
     expect(tokenMap.get(key), same(value));
   }
 }