cleanup and improve A/B fasta test harness

pkg/front_end/tool/fasta/standard_deviation.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';
 
 double average(List<double> elapsedTimes) {
   return elapsedTimes.reduce((v, e) => v + e) / elapsedTimes.length;
 }
 
 double standardDeviation(double mean, List<double> elapsedTimes) {
   double numerator = 0.0;
   for (double elapseTime in elapsedTimes) {
     numerator += (elapseTime - mean) * (elapseTime - mean);
   }
   double stdDev = sqrt(numerator / (elapsedTimes.length - 1));
   return stdDev;
 }
 
 double standardDeviationOfTheMean(List<double> elapsedTimes, double stdDev) {
   return stdDev / sqrt(elapsedTimes.length);
 }