devicelab: started averaging startup time for ios test (#64978)

dev/devicelab/lib/tasks/perf_tests.dart

@@ -341,6 +341,23 @@ TaskFunction createFramePolicyIntegrationTest() {
   };
 }
 
+Map<String, dynamic> _average(List<Map<String, dynamic>> results, int iterations) {
+  final Map<String, dynamic> tally = <String, dynamic>{};
+  for (final Map<String, dynamic> item in results) {
+    item.forEach((String key, dynamic value) {
+      if (tally.containsKey(key)) {
+        tally[key] = (tally[key] as int) + (value as int);
+      } else {
+        tally[key] = value;
+      }
+    });
+  }
+  tally.forEach((String key, dynamic value) {
+    tally[key] = (value as int) ~/ iterations;
+  });
+  return tally;
+}
+
 /// Measure application startup performance.
 class StartupTest {
   const StartupTest(this.testDirectory, { this.reportMetrics = true });
@@ -353,6 +370,9 @@ class StartupTest {
       final String deviceId = (await devices.workingDevice).deviceId;
       await flutter('packages', options: <String>['get']);
 
+      const int iterations = 3;
+      final List<Map<String, dynamic>> results = <Map<String, dynamic>>[];
+      for (int i = 0; i < iterations; ++i) {
         await flutter('run', options: <String>[
           '--verbose',
           '--profile',
@@ -363,11 +383,15 @@ class StartupTest {
         final Map<String, dynamic> data = json.decode(
           file('$testDirectory/build/start_up_info.json').readAsStringSync(),
         ) as Map<String, dynamic>;
+        results.add(data);
+      }
+
+      final Map<String, dynamic> averageResults = _average(results, iterations);
 
       if (!reportMetrics)
-        return TaskResult.success(data);
+        return TaskResult.success(averageResults);
 
-      return TaskResult.success(data, benchmarkScoreKeys: <String>[
+      return TaskResult.success(averageResults, benchmarkScoreKeys: <String>[
         'timeToFirstFrameMicros',
         'timeToFirstFrameRasterizedMicros',
       ]);