scroll [nfc]: Move scroll-to-bottom logic into our ScrollPosition subclass

This makes a more comfortable home for extending this logic,
because it naturally uses a lot of ScrollPosition members.

To make the logic fit in better on this class, also loosen its
assumptions slightly, allowing maxScrollExtent to be nonzero.
It's still expected not to change, though -- fixing that is a more
complex job, and will come over the remainder of this commit series.

Author: gnprice

lib/widgets/message_list.dart

@@ -1,5 +1,3 @@
-import 'dart:math';
-
 import 'package:collection/collection.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_color_models/flutter_color_models.dart';
@@ -696,14 +694,7 @@ class ScrollToBottomButton extends StatelessWidget {
   final MessageListScrollController scrollController;
 
   void _scrollToBottom() {
-    final target = 0.0;
-    final distance = target - scrollController.position.pixels;
-    final durationMsAtSpeedLimit = (1000 * distance / 8000).ceil();
-    final durationMs = max(300, durationMsAtSpeedLimit);
-    scrollController.animateTo(
-      target,
-      duration: Duration(milliseconds: durationMs),
-      curve: Curves.linear);
+    scrollController.position.scrollToEnd();
   }
 
   @override

lib/widgets/scrolling.dart

@@ -167,6 +167,26 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
 
     return !changed;
   }
+
+  /// Scroll the position smoothly to the end of the scrollable content.
+  ///
+  /// This method only works well if [maxScrollExtent] is accurate
+  /// and does not change during the animation.
+  /// (For example, this works if there is no content in forward slivers,
+  /// so that [maxScrollExtent] is always zero.)
+  /// The animation will attempt to travel to the value [maxScrollExtent] had
+  /// at the start of the animation, even if that ends up being more or less far
+  /// than the actual extent of the content.
+  void scrollToEnd() {
+    final target = maxScrollExtent;
+    final distance = target - pixels;
+    final durationMsAtSpeedLimit = (1000 * distance / 8000).ceil();
+    final durationMs = math.max(300, durationMsAtSpeedLimit);
+    animateTo(
+      target,
+      duration: Duration(milliseconds: durationMs),
+      curve: Curves.linear);
+  }
 }
 
 /// A version of [ScrollController] adapted for the Zulip message list.