Port sky-drawer to Dart

sky/framework/animation/controller.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.
--->
-<import src="timer.sky" as="AnimationTimer" />
-<script>
-module.exports = class AnimationController {
-  constructor(delegate) {
-    this.delegate_ = delegate;
-    this.timer_ = new AnimationTimer(this);
-    this.begin_ = 0;
-    this.end_ = 0;
-    this.curve_ = null;
-    this.isAnimating = false;
-    Object.preventExtensions(this);
-  }
-
-  start(options) {
-    this.begin_ = options.begin;
-    this.end_ = options.end;
-    this.curve_ = options.curve;
-    this.isAnimating = true;
-    this.timer_.start(options.duration);
-  }
-
-  stop() {
-    this.isAnimating = false;
-    this.timer_.stop();
-  }
-
-  positionForTime_(t) {
-    // Explicitly finish animations at |this.end_| in case the curve isn't an
-    // exact numerical transform.
-    if (t == 1)
-      return this.end_;
-    var curvedTime = this.curve_.transform(t);
-    var begin = this.begin_;
-    return begin + (this.end_ - begin) * curvedTime;
-  }
-
-  updateAnimation(t) {
-    this.delegate_.updateAnimation(this.positionForTime_(t));
-  }
-};
-</script>