Re-apply 'Add currentSystemFrameTimeStamp to SchedulerBinding'

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

@@ -853,6 +853,22 @@ mixin SchedulerBinding on BindingBase, ServicesBinding {
   }
   Duration _currentFrameTimeStamp;
 
+  /// The raw time stamp as provided by the engine to [Window.onBeginFrame]
+  /// for the frame currently being processed.
+  ///
+  /// Unlike [currentFrameTimeStamp], this time stamp is neither adjusted to
+  /// offset when the epoch started nor scaled to reflect the [timeDilation] in
+  /// the current epoch.
+  ///
+  /// On most platforms, this is a more or less arbitrary value, and should
+  /// generally be ignored. On Fuchsia, this corresponds to the system-provided
+  /// presentation time, and can be used to ensure that animations running in
+  /// different processes are synchronized.
+  Duration get currentSystemFrameTimeStamp {
+    assert(_lastRawTimeStamp != null);
+    return _lastRawTimeStamp;
+  }
+
   int _debugFrameNumber = 0;
   String _debugBanner;
   bool _ignoreNextEngineDrawFrame = false;

packages/flutter/test/scheduler/scheduler_test.dart

@@ -10,6 +10,7 @@ import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
 
 import '../flutter_test_alternative.dart';
+import 'scheduler_tester.dart';
 
 class TestSchedulerBinding extends BindingBase with ServicesBinding, SchedulerBinding {
   final Map<String, List<Map<String, dynamic>>> eventsDispatched = <String, List<Map<String, dynamic>>>{};
@@ -148,4 +149,36 @@ void main() {
     expect(event['build'], 5000);
     expect(event['raster'], 4000);
   });
+
+  test('currentSystemFrameTimeStamp is the raw timestamp', () {
+    Duration lastTimeStamp;
+    Duration lastSystemTimeStamp;
+
+    void frameCallback(Duration timeStamp) {
+      expect(timeStamp, scheduler.currentFrameTimeStamp);
+      lastTimeStamp = scheduler.currentFrameTimeStamp;
+      lastSystemTimeStamp = scheduler.currentSystemFrameTimeStamp;
+    }
+
+    scheduler.scheduleFrameCallback(frameCallback);
+    tick(const Duration(seconds: 2));
+    expect(lastTimeStamp, Duration.zero);
+    expect(lastSystemTimeStamp, const Duration(seconds: 2));
+
+    scheduler.scheduleFrameCallback(frameCallback);
+    tick(const Duration(seconds: 4));
+    expect(lastTimeStamp, const Duration(seconds: 2));
+    expect(lastSystemTimeStamp, const Duration(seconds: 4));
+
+    timeDilation = 2;
+    scheduler.scheduleFrameCallback(frameCallback);
+    tick(const Duration(seconds: 6));
+    expect(lastTimeStamp, const Duration(seconds: 2)); // timeDilation calls SchedulerBinding.resetEpoch
+    expect(lastSystemTimeStamp, const Duration(seconds: 6));
+
+    scheduler.scheduleFrameCallback(frameCallback);
+    tick(const Duration(seconds: 8));
+    expect(lastTimeStamp, const Duration(seconds: 3)); // 2s + (8 - 6)s / 2
+    expect(lastSystemTimeStamp, const Duration(seconds: 8));
+  });
 }