add previous token field

pkg/front_end/tool/fasta/compile.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:math';
-
 import 'package:front_end/src/fasta/outline.dart' as outline;
 
 import 'standard_deviation.dart';
 
 const int iterations = const int.fromEnvironment("iterations", defaultValue: 1);
 
 main(List<String> arguments) async {
   // Timing results for each iteration
   List<double> elapseTimes = <double>[];
 
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   // Calculate the standard deviation of the mean
   double stdDevOfTheMean = standardDeviationOfTheMean(warmTimes, stdDev);
 
   print('Summary:');
   print('  Elapse times: $elapseTimes');
   print('  Cold start (first run): ${elapseTimes[0]}');
   print('  Warm run average (runs #4 to #$iterations): $mean');
   print('  Warm run standard deviation of the mean: $stdDevOfTheMean');
 }