Add Robert Penner’s easing functions (#25788)

* Add Robert Penner’s easing functions

These easing functions are the most popular functions in use on the web
and other platforms. Including them in Flutter will encourage users to
create more engaging and attractive animations.

packages/flutter/test/animation/curves_test.dart

@@ -45,16 +45,42 @@ void main() {
   test('Curve is continuous', () {
     assertMaximumSlope(Curves.linear, 20.0);
     assertMaximumSlope(Curves.decelerate, 20.0);
+    assertMaximumSlope(Curves.fastOutSlowIn, 20.0);
     assertMaximumSlope(Curves.bounceIn, 20.0);
     assertMaximumSlope(Curves.bounceOut, 20.0);
     assertMaximumSlope(Curves.bounceInOut, 20.0);
     assertMaximumSlope(Curves.elasticOut, 20.0);
     assertMaximumSlope(Curves.elasticInOut, 20.0);
     assertMaximumSlope(Curves.ease, 20.0);
+
     assertMaximumSlope(Curves.easeIn, 20.0);
+    assertMaximumSlope(Curves.easeInSine, 20.0);
+    assertMaximumSlope(Curves.easeInQuad, 20.0);
+    assertMaximumSlope(Curves.easeInCubic, 20.0);
+    assertMaximumSlope(Curves.easeInQuart, 20.0);
+    assertMaximumSlope(Curves.easeInQuint, 20.0);
+    assertMaximumSlope(Curves.easeInExpo, 20.0);
+    assertMaximumSlope(Curves.easeInCirc, 20.0);
+
     assertMaximumSlope(Curves.easeOut, 20.0);
+    assertMaximumSlope(Curves.easeOutSine, 20.0);
+    assertMaximumSlope(Curves.easeOutQuad, 20.0);
+    assertMaximumSlope(Curves.easeOutCubic, 20.0);
+    assertMaximumSlope(Curves.easeOutQuart, 20.0);
+    assertMaximumSlope(Curves.easeOutQuint, 20.0);
+    assertMaximumSlope(Curves.easeOutExpo, 20.0);
+    assertMaximumSlope(Curves.easeOutCirc, 20.0);
+
+    // Curves.easeInOutExpo is discontinuous at its midpoint, so not included
+    // here
+
     assertMaximumSlope(Curves.easeInOut, 20.0);
-    assertMaximumSlope(Curves.fastOutSlowIn, 20.0);
+    assertMaximumSlope(Curves.easeInOutSine, 20.0);
+    assertMaximumSlope(Curves.easeInOutQuad, 20.0);
+    assertMaximumSlope(Curves.easeInOutCubic, 20.0);
+    assertMaximumSlope(Curves.easeInOutQuart, 20.0);
+    assertMaximumSlope(Curves.easeInOutQuint, 20.0);
+    assertMaximumSlope(Curves.easeInOutCirc, 20.0);
   });
 
   void expectStaysInBounds(Curve curve) {
@@ -115,6 +141,23 @@ void main() {
     expect(bounds[1], greaterThan(1.0));
   });
 
+  test('Back overshoots its bounds', () {
+    expect(Curves.easeInBack, hasOneLineDescription);
+    expect(Curves.easeOutBack, hasOneLineDescription);
+    expect(Curves.easeInOutBack, hasOneLineDescription);
+
+    List<double> bounds;
+    bounds = estimateBounds(Curves.easeInBack);
+    expect(bounds[0], lessThan(0.0));
+    expect(bounds[1], lessThanOrEqualTo(1.0));
+    bounds = estimateBounds(Curves.easeOutBack);
+    expect(bounds[0], greaterThanOrEqualTo(0.0));
+    expect(bounds[1], greaterThan(1.0));
+    bounds = estimateBounds(Curves.easeInOutBack);
+    expect(bounds[0], lessThan(0.0));
+    expect(bounds[1], greaterThan(1.0));
+  });
+
   test('Decelerate does so', () {
     expect(Curves.decelerate, hasOneLineDescription);