What is the optimal render loop in Dart 2?

Well, after a month of beating my face against this, I finally figured out the right question and that got me to this:Flutter Layers / Raw

// 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.// This example shows how to perform a simple animation using the raw interface// to the engine.import 'dart:math' as math;import 'dart:typed_data';import 'dart:ui' as ui;void beginFrame(Duration timeStamp) {  // The timeStamp argument to beginFrame indicates the timing information we  // should use to clock our animations. It's important to use timeStamp rather  // than reading the system time because we want all the parts of the system to  // coordinate the timings of their animations. If each component read the  // system clock independently, the animations that we processed later would be  // slightly ahead of the animations we processed earlier.  // PAINT  final ui.Rect paintBounds = ui.Offset.zero & (ui.window.physicalSize / ui.window.devicePixelRatio);  final ui.PictureRecorder recorder = ui.PictureRecorder();  final ui.Canvas canvas = ui.Canvas(recorder, paintBounds);  canvas.translate(paintBounds.width / 2.0, paintBounds.height / 2.0);  // Here we determine the rotation according to the timeStamp given to us by  // the engine.  final double t = timeStamp.inMicroseconds / Duration.microsecondsPerMillisecond / 1800.0;  canvas.rotate(math.pi * (t % 1.0));  canvas.drawRect(ui.Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),                  ui.Paint()..color = const ui.Color.fromARGB(255, 0, 255, 0));  final ui.Picture picture = recorder.endRecording();  // COMPOSITE  final double devicePixelRatio = ui.window.devicePixelRatio;  final Float64List deviceTransform = Float64List(16)    ..[0] = devicePixelRatio    ..[5] = devicePixelRatio    ..[10] = 1.0    ..[15] = 1.0;  final ui.SceneBuilder sceneBuilder = ui.SceneBuilder()    ..pushTransform(deviceTransform)    ..addPicture(ui.Offset.zero, picture)    ..pop();  ui.window.render(sceneBuilder.build());  // After rendering the current frame of the animation, we ask the engine to  // schedule another frame. The engine will call beginFrame again when its time  // to produce the next frame.  ui.window.scheduleFrame();}void main() {  ui.window.onBeginFrame = beginFrame;  ui.window.scheduleFrame();}