Port sky-drawer to Dart

sky/framework/animation/curves.dart

+// 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.
+
+double _evaluateCubic(double a, double b, double m) {
+  // TODO(abarth): Would Math.pow be faster?
+  return 3 * a * (1 - m) * (1 - m) * m + 3 * b * (1 - m) * m * m + m * m * m;
+}
+
+const double _kCubicErrorBound = 0.001;
+
+abstract class Curve {
+  double transform(double t);
+}
+
+class Linear implements Curve {
+  const Linear();
+
+  double transform(double t) {
+    return t;
+  }
+}
+
+class Cubic implements Curve {
+  final double a;
+  final double b;
+  final double c;
+  final double d;
+
+  const Cubic(this.a, this.b, this.c, this.d);
+
+  double transform(double t) {
+    double start = 0.0;
+    double end = 1.0;
+    while (true) {
+      double midpoint = (start + end) / 2;
+      double estimate = _evaluateCubic(a, c, midpoint);
+
+      if ((t - estimate).abs() < _kCubicErrorBound)
+        return _evaluateCubic(b, d, midpoint);
+
+      if (estimate < t)
+        start = midpoint;
+      else
+        end = midpoint;
+    }
+  }
+}
+
+const Linear linear = const Linear();
+const Cubic ease = const Cubic(0.25, 0.1, 0.25, 1.0);
+const Cubic easeIn = const Cubic(0.42, 0.0, 1.0, 1.0);
+const Cubic easeOut = const Cubic(0.0, 0.0, 0.58, 1.0);
+const Cubic easeInOut = const Cubic(0.42, 0.0, 0.58, 1.0);