Clean up scroll behaviour.

This makes ScrollBehavior agnostic about the kind of scroll.
This also means ExtentScrollBehavior's comment about being linear is
more relevant (previously it was true of all scroll behaviours).

packages/flutter/lib/src/widgets/scroll_behavior.dart

@@ -10,27 +10,36 @@ const double _kSecondsPerMillisecond = 1000.0;
 const double _kScrollDrag = 0.025;
 
 /// An interface for controlling the behavior of scrollable widgets.
-abstract class ScrollBehavior {
+///
+/// The type argument T is the type that describes the scroll offset.
+/// The type argument U is the type that describes the scroll velocity.
+abstract class ScrollBehavior<T, U> {
   /// Returns a simulation that propels the scrollOffset.
   ///
   /// This function is called when a drag gesture ends.
-  Simulation createFlingScrollSimulation(double position, double velocity) => null;
+  ///
+  /// Returns null if the behavior is to do nothing.
+  Simulation createFlingScrollSimulation(T position, U velocity) => null;
 
   /// Returns an animation that ends at the snap offset.
   ///
-  /// This function is called when a drag gesture ends and toSnapOffset is specified.
-  Simulation createSnapScrollSimulation(double startOffset, double endOffset, double startVelocity, double endVelocity) => null;
+  /// This function is called when a drag gesture ends and a
+  /// [SnapOffsetCallback] is specified for the scrollable.
+  ///
+  /// Returns null if the behavior is to do nothing.
+  Simulation createSnapScrollSimulation(T startOffset, T endOffset, U startVelocity, U endVelocity) => null;
 
   /// Returns the scroll offset to use when the user attempts to scroll
   /// from the given offset by the given delta.
-  double applyCurve(double scrollOffset, double scrollDelta);
+  T applyCurve(T scrollOffset, T scrollDelta) => scrollOffset;
 
   /// Whether this scroll behavior currently permits scrolling
   bool get isScrollable => true;
 }
 
-/// A scroll behavior for a scrollable widget with linear extent.
-abstract class ExtentScrollBehavior extends ScrollBehavior {
+/// A scroll behavior for a scrollable widget with linear extent (i.e.
+/// that only scrolls along one axis).
+abstract class ExtentScrollBehavior extends ScrollBehavior<double, double> {
   ExtentScrollBehavior({ double contentExtent: 0.0, double containerExtent: 0.0 })
     : _contentExtent = contentExtent, _containerExtent = containerExtent;