Add script for profiling startup using flutter trace on Android

This will work after the next Engine roll.  Currently it requires
a fix to tracing which isn't in the released engine.

Next steps are to make this work on a bot, and to add iOS support.

@abarth @chinmaygarde

dev/profile_startup.dart

@@ -0,0 +1,95 @@
+#!/usr/bin/env dart
+// Copyright 2015 The Chromium Authors. 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:io';
+import 'dart:async';
+import 'dart:convert';
+
+const int ITERATIONS = 3;
+
+String runWithLoggingSync(List<String> cmd, {
+  bool checked: true,
+  String workingDirectory
+}) {
+  ProcessResult results =
+      Process.runSync(cmd[0], cmd.getRange(1, cmd.length).toList(), workingDirectory: workingDirectory);
+  if (results.exitCode != 0) {
+    String errorDescription = 'Error code ${results.exitCode} '
+        'returned when attempting to run command: ${cmd.join(' ')}';
+    print(errorDescription);
+    if (results.stderr.length > 0)
+      print('Errors logged: ${results.stderr.trim()}');
+    if (checked)
+      throw errorDescription;
+  }
+  if (results.stdout.trim().isNotEmpty)
+    print(results.stdout.trim());
+  return results.stdout;
+}
+
+double timeToFirstFrame(trace) {
+  // TODO(eseidel): Sort! Events are not guarenteed to be in timestamp order.
+  List events = trace['traceEvents'];
+  int firstTimeStamp = events[0]['ts'].toInt();
+  var firstSwap = events.firstWhere((e) => e['name'] == 'NativeViewGLSurfaceEGL:RealSwapBuffers');
+  int swapStart = firstSwap['ts'].toInt();
+  int swapEnd = swapStart + firstSwap['dur'].toInt();
+  return (swapEnd - firstTimeStamp) / 1000; // microseconds to milliseconds.
+}
+
+Future<double> test(String tracesDir, String projectPath, int runNumber) async {
+  // If we used package:path we could grab the basename of project_path
+  // and include that in the trace_name.
+  String tracePath = "${tracesDir}/trace_$runNumber.json";
+  runWithLoggingSync([
+    'flutter',
+    'start',
+    '--trace-startup'
+  ], workingDirectory: projectPath);
+  await new Future.delayed(const Duration(seconds: 2), () => "");
+  runWithLoggingSync([
+    'flutter',
+    'trace',
+    '--stop',
+    '--out=${tracePath}'
+  ], workingDirectory: projectPath);
+
+  JsonDecoder decoder = new JsonDecoder();
+  String contents = await new File(tracePath).readAsString();
+  Map data = await decoder.convert(contents);
+  return timeToFirstFrame(data);
+}
+
+// package:statistics has slightly nicer ones of these.
+double mean(List<double> times) {
+  return times.reduce((a,b) => a + b) / times.length;
+}
+
+double median(List<double> times) {
+  times.sort();
+  return times[times.length ~/ 2];
+}
+
+main(List<String> args) async {
+  // We could do much more sophisticated things if we used package:args.
+  if (args.length < 1) {
+    print("Usage: profile_startup.dart PROJECT_PATH\n");
+    print("PROJECT_PATH required.");
+    return 1;
+  }
+  String projectPath = args[0];
+  String traces_dir = '/tmp';
+
+  List<double> times = [];
+  print("$ITERATIONS runs using $projectPath:");
+  for (var x = 0; x < ITERATIONS; x++) {
+    int runNumber = x + 1;
+    double time = await test(traces_dir, projectPath, runNumber);
+    print(" ${runNumber.toString().padLeft(2)} $time");
+    times.add(time);
+  }
+  print("mean: ${mean(times)}");
+  print("median: ${median(times)}");
+}