add previous token field

pkg/front_end/test/scanner_fasta_test.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 'package:front_end/src/fasta/analyzer/token_utils.dart';
 import 'package:front_end/src/fasta/scanner/error_token.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/keyword.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/string_scanner.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/token.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/token_constants.dart' as fasta;
@@ skipping 176 matching lines @@
     super.test_mismatched_opener();
   }
 
   @override
   @failingTest
   void test_scriptTag_withArgs() {
     // TODO(paulberry,ahe): script tags are needed by analyzer.
     super.test_scriptTag_withArgs();
   }
 
+  void test_next_previous() {
+    const source = 'int a; /*1*/ /*2*/ /*3*/ B f(){if (a < 2) {}}';
+    fasta.Token token =
+        new fasta.StringScanner(source, includeComments: true).tokenize();
+    while (!token.isEof) {
+      expect(token.next.previousToken, token);
+      fasta.Token commentToken = token.precedingComments;
+      while (commentToken != null) {
+        if (commentToken.next != null) {
+          expect(commentToken.next.previousToken, commentToken);
+        }
+        commentToken = commentToken.next;
+      }
+      token = token.next;
+    }
+  }
+
   @override
   @failingTest
   void test_scriptTag_withoutSpace() {
     // TODO(paulberry,ahe): script tags are needed by analyzer.
     super.test_scriptTag_withoutSpace();
   }
 
   @override
   @failingTest
   void test_scriptTag_withSpace() {
@@ skipping 272 matching lines @@
   final ErrorListener _listener;
 
   ToAnalyzerTokenStreamConverter_WithListener(this._listener);
 
   @override
   void reportError(
       ScannerErrorCode errorCode, int offset, List<Object> arguments) {
     _listener.errors.add(new TestError(offset, errorCode, arguments));
   }
 }