Fix SpringSimulation formula for underdamping (#165017)

Fix https://github.com/flutter/flutter/issues/163858

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packages/flutter/lib/src/physics/spring_simulation.dart

@@ -364,7 +364,7 @@ class _UnderdampedSolution implements _SpringSolution {
     final double w =
         math.sqrt(4.0 * spring.mass * spring.stiffness - spring.damping * spring.damping) /
         (2.0 * spring.mass);
-    final double r = -(spring.damping / 2.0 * spring.mass);
+    final double r = -(spring.damping / 2.0 / spring.mass);
     final double c1 = distance;
     final double c2 = (velocity - r * distance) / w;
     return _UnderdampedSolution.withArgs(w, r, c1, c2);

packages/flutter/test/physics/spring_simulation_test.dart

@@ -36,6 +36,39 @@ void main() {
     expect(snappingSimulation.dx(time), 0);
   });
 
+  test('SpringSimulation results are continuous near critical damping', () {
+    // Regression test for https://github.com/flutter/flutter/issues/163858
+    const double time = 0.4;
+    const double stiffness = 0.4;
+    const double mass = 0.4;
+    final SpringSimulation critical = SpringSimulation(
+      SpringDescription.withDampingRatio(stiffness: stiffness, mass: mass),
+      0,
+      1,
+      0,
+    );
+    expect(critical.x(time), moreOrLessEquals(0.06155, epsilon: 0.01));
+    expect(critical.dx(time), moreOrLessEquals(0.2681, epsilon: 0.01));
+
+    final SpringSimulation slightlyOver = SpringSimulation(
+      SpringDescription.withDampingRatio(stiffness: stiffness, mass: mass, ratio: 1 + 1e-3),
+      0,
+      1,
+      0,
+    );
+    expect(slightlyOver.x(time), moreOrLessEquals(0.06155, epsilon: 0.01));
+    expect(slightlyOver.dx(time), moreOrLessEquals(0.2681, epsilon: 0.01));
+
+    final SpringSimulation slightlyUnder = SpringSimulation(
+      SpringDescription.withDampingRatio(stiffness: stiffness, mass: mass, ratio: 1 - 1e-3),
+      0,
+      1,
+      0,
+    );
+    expect(slightlyUnder.x(time), moreOrLessEquals(0.06155, epsilon: 0.01));
+    expect(slightlyUnder.dx(time), moreOrLessEquals(0.2681, epsilon: 0.01));
+  });
+
   group('SpringDescription.withDurationAndBounce', () {
     test('creates spring with expected results', () {
       final SpringDescription spring = SpringDescription.withDurationAndBounce(bounce: 0.3);