Port sky-drawer to Dart

sky/framework/animation/timer.sky

-<!--
-// 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.
--->
-<script>
-module.exports = class AnimationTimer {
-  constructor(delegate) {
-    this.delegate_ = delegate;
-    this.startTime_ = 0;
-    this.duration_ = 0;
-    this.animationId_ = 0;
-    Object.preventExtensions(this);
-  }
-
-  start(duration) {
-    if (this.animationId_)
-      this.stop();
-    this.duration_ = duration;
-    this.scheduleTick_();
-  }
-
-  stop() {
-    cancelAnimationFrame(this.animationId_);
-    this.startTime_ = 0;
-    this.duration_ = 0;
-    this.animationId_ = 0;
-  }
-
-  scheduleTick_() {
-    if (this.animationId_)
-      throw new Error("Tick already scheduled.");
-    this.animationId_ = requestAnimationFrame(this.tick_.bind(this));
-  }
-
-  tick_(timeStamp) {
-    this.animationId_ = 0;
-    if (!this.startTime_)
-      this.startTime_ = timeStamp;
-    var elapsedTime = timeStamp - this.startTime_;
-    var t = Math.max(0, Math.min(1, elapsedTime / this.duration_));
-    if (t < 1)
-      this.scheduleTick_();
-    else
-      this.stop();
-    this.delegate_.updateAnimation(t);
-  }
-};
-</script>