export missed transition count to the dashboard (#7426)

dev/devicelab/lib/tasks/gallery.dart

@@ -5,6 +5,7 @@
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
+import 'dart:math' as math;
 
 import '../framework/adb.dart';
 import '../framework/framework.dart';
@@ -42,10 +43,10 @@ class GalleryTransitionTest {
 
     // Route paths contains slashes, which Firebase doesn't accept in keys, so we
     // remove them.
-    Map<String, dynamic> original = JSON.decode(file(
+    Map<String, List<int>> original = JSON.decode(file(
             '${galleryDirectory.path}/build/transition_durations.timeline.json')
         .readAsStringSync());
-    Map<String, dynamic> transitions = new Map<String, dynamic>.fromIterable(
+    Map<String, List<int>> transitions = new Map<String, List<int>>.fromIterable(
         original.keys,
         key: (String key) => key.replaceAll('/', ''),
         value: (String key) => original[key]);
@@ -54,10 +55,12 @@ class GalleryTransitionTest {
 
     Map<String, dynamic> data = <String, dynamic>{
       'transitions': transitions,
+      'missed_transition_count': _countMissedTransitions(transitions),
     };
     data.addAll(summary);
 
     return new TaskResult.success(data, benchmarkScoreKeys: <String>[
+      'missed_transition_count',
       'average_frame_build_time_millis',
       'worst_frame_build_time_millis',
       'missed_frame_build_budget_count',
@@ -67,3 +70,16 @@ class GalleryTransitionTest {
     ]);
   }
 }
+
+int _countMissedTransitions(Map<String, List<int>> transitions) {
+  const int _kTransitionBudget = 100000; // µs
+  int count = 0;
+  transitions.forEach((String demoName, List<int> durations) {
+    int longestDuration = durations.reduce(math.max);
+    if (longestDuration > _kTransitionBudget) {
+      print('$demoName missed transition time budget ($longestDuration µs > $_kTransitionBudget µs)');
+      count++;
+    }
+  });
+  return count;
+}