Don't flush touch events during first launch frame pre-warm (#13990)

* Lock events during runApp's warm up frame

* move to scheduler binding

* let the one scheduleWarmUpFrame api always lock

* tweak test

packages/flutter/lib/src/foundation/binding.dart

@@ -161,21 +161,25 @@ abstract class BindingBase {
   /// callback's future completes.
   ///
   /// This causes input lag and should therefore be avoided when possible. It is
-  /// primarily intended for development features, in particular to allow
-  /// [reassembleApplication] to block input while it walks the tree (which it
-  /// partially does asynchronously).
+  /// primarily intended for use during non-user-interactive time such as to
+  /// allow [reassembleApplication] to block input while it walks the tree
+  /// (which it partially does asynchronously).
   ///
   /// The [Future] returned by the `callback` argument is returned by [lockEvents].
   @protected
   Future<Null> lockEvents(Future<Null> callback()) {
+    developer.Timeline.startSync('Lock events');
+
     assert(callback != null);
     _lockCount += 1;
     final Future<Null> future = callback();
     assert(future != null, 'The lockEvents() callback returned null; it should return a Future<Null> that completes when the lock is to expire.');
     future.whenComplete(() {
       _lockCount -= 1;
-      if (!locked)
+      if (!locked) {
+        developer.Timeline.finishSync();
         unlocked();
+      }
     });
     return future;
   }

packages/flutter/lib/src/scheduler/binding.dart

@@ -666,6 +666,8 @@ abstract class SchedulerBinding extends BindingBase with ServicesBinding {
   /// This is used during application startup so that the first frame (which is
   /// likely to be quite expensive) gets a few extra milliseconds to run.
   ///
+  /// Locks events dispatching until the scheduled frame has completed.
+  ///
   /// If a frame has already been scheduled with [scheduleFrame] or
   /// [scheduleForcedFrame], this call may delay that frame.
   ///
@@ -677,8 +679,9 @@ abstract class SchedulerBinding extends BindingBase with ServicesBinding {
     if (_warmUpFrame || schedulerPhase != SchedulerPhase.idle)
       return;
 
-    final bool hadScheduledFrame = _hasScheduledFrame;
     _warmUpFrame = true;
+    Timeline.startSync('Warm-up frame');
+    final bool hadScheduledFrame = _hasScheduledFrame;
     // We use timers here to ensure that microtasks flush in between.
     Timer.run(() {
       assert(_warmUpFrame);
@@ -700,6 +703,13 @@ abstract class SchedulerBinding extends BindingBase with ServicesBinding {
       if (hadScheduledFrame)
         scheduleFrame();
     });
+
+    // Lock events so touch events etc don't insert themselves until the
+    // scheduled frame has finished.
+    lockEvents(() async {
+      await endOfFrame;
+      Timeline.finishSync();
+    });
   }
 
   Duration _firstRawTimeStampInEpoch;

packages/flutter/test/scheduler/scheduler_test.dart

@@ -92,11 +92,13 @@ void main() {
 
   test('2 calls to scheduleWarmUpFrame just schedules it once', () {
     final List<VoidCallback> timerQueueTasks = <VoidCallback>[];
+    bool taskExecuted = false;
     runZoned(
       () {
         // Run it twice without processing the queued tasks.
         scheduler.scheduleWarmUpFrame();
         scheduler.scheduleWarmUpFrame();
+        scheduler.scheduleTask(() { taskExecuted = true; }, Priority.touch);
       },
       zoneSpecification: new ZoneSpecification(
         createTimer: (Zone self, ZoneDelegate parent, Zone zone, Duration duration, void f()) {
@@ -107,7 +109,9 @@ void main() {
       ),
     );
 
-    // A single call to scheduleWarmUpFrame queues up 2 Timer tasks.
+    // scheduleWarmUpFrame scheduled 2 Timers, scheduleTask scheduled 0 because
+    // events are locked.
     expect(timerQueueTasks.length, 2);
+    expect(taskExecuted, false);
   });
 }