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 - 1));
+  return stdDev;
+}
+
+double standardDeviationOfTheMean(List<double> elapsedTimes, double stdDev) {
+  return stdDev / sqrt(elapsedTimes.length);
+}