A/B fasta performance measurement harness

pkg/front_end/tool/fasta/standard_deviation.dart

+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);

[Paul Berry, 2017/03/03 18:08:53]

This should be:

`stdDev = sqrt(numerator / (elapsedTimes.length - 1));`.

The reason is complicated--see
https://en.wikipedia.org/wiki/Bessel%27s_correction for more information.

[danrubel, 2017/03/03 22:46:56]

Done.

+  return stdDev;
+}
+
+double standardDeviationOfTheMean(List<double> elapsedTimes, double stdDev) {
+  return stdDev / sqrt(elapsedTimes.length);

[Paul Berry, 2017/03/03 18:08:53]

This one is correct.

+}