scroll [nfc]: Introduce SimulationDrivenScrollActivity

In order to implement the "scroll to bottom" button in a way that
behaves well when scrolling to the growth end of a sliver -- in
particular, when scrolling to the end of the message list after we
split it into back-to-back slivers -- we'll want some differences
from the behavior provided by DrivenScrollActivity, which we've used
up until now (originally via the `animateTo` method).

That calls for our own ScrollActivity subclass, ScrollToEndActivity.
We'll want most of the same behavior as DrivenScrollActivity, with
just a couple of changes; but one of the places we want to change
isn't among the places that DrivenScrollActivity exposes for
subclassing.  So add this class, based on DrivenScrollActivity but
with a customization point added in the additional spot we'll need.

Originally there were two additional customization points needed.
After first drafting this change, I sent those upstream as two PRs:
 flutter/flutter#166730
 flutter/flutter#166731

The first one has already landed in DrivenScrollActivity: the
applyMoveTo method now overridden by ScrollToEndActivity in a
previous commit.  The other one, a `.simulation` constructor, is
pending.  A "TODO(upstream)" comment points to that PR, because
once it also merges we can dispense with this class.

Author: gnprice

lib/widgets/scrolling.dart

@@ -249,6 +249,78 @@ class RenderCustomPaintOrderViewport extends RenderViewport {
   }
 }
 
+/// Like [DrivenScrollActivity], but accepting a general [Simulation].
+// TODO(upstream): https://github.com/flutter/flutter/pull/166731
+class SimulationDrivenScrollActivity extends ScrollActivity {
+  SimulationDrivenScrollActivity.simulation(
+    super.delegate,
+    Simulation simulation, {
+    required TickerProvider vsync,
+  }) {
+    _controller = AnimationController.unbounded(vsync: vsync)
+      ..addListener(_tick)
+      ..animateWith(simulation).whenComplete(_end);
+  }
+
+  late final AnimationController _controller;
+  bool _isDisposed = false;
+
+  void _tick() {
+    if (!applyMoveTo(_controller.value)) {
+      delegate.goIdle();
+    }
+  }
+
+  /// Move the position to the given location.
+  ///
+  /// Returns true if the position was fully applied,
+  /// false if there was overflow.
+  bool applyMoveTo(double value) {
+    return delegate.setPixels(value).abs() < precisionErrorTolerance;
+  }
+
+  void _end() {
+    if (!_isDisposed) {
+      delegate.goBallistic(velocity);
+    }
+  }
+
+  @override
+  void dispatchOverscrollNotification(
+    ScrollMetrics metrics,
+    BuildContext context,
+    double overscroll,
+  ) {
+    OverscrollNotification(
+      metrics: metrics,
+      context: context,
+      overscroll: overscroll,
+      velocity: velocity,
+    ).dispatch(context);
+  }
+
+  @override
+  bool get shouldIgnorePointer => true;
+
+  @override
+  bool get isScrolling => true;
+
+  @override
+  double get velocity => _controller.velocity;
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    _isDisposed = true;
+    super.dispose();
+  }
+
+  @override
+  String toString() {
+    return '${describeIdentity(this)}($_controller)';
+  }
+}
+
 /// An activity that animates a scroll view smoothly to its end.
 ///
 /// In particular this drives the "scroll to bottom" button