Fix PageableList demo

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

@@ -131,11 +131,19 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
     );
   }
 
-  ScrollBehavior createScrollBehavior() {
-    return config.itemsWrap ? new UnboundedBehavior() : new OverscrollBehavior();
+  UnboundedBehavior _unboundedBehavior;
+  OverscrollBehavior _overscrollBehavior;
+
+  ExtentScrollBehavior get scrollBehavior {
+    if (config.itemsWrap) {
+      _unboundedBehavior ??= new UnboundedBehavior();
+      return _unboundedBehavior;
+    }
+    _overscrollBehavior ??= new OverscrollBehavior();
+    return _overscrollBehavior;
   }
 
-  ExtentScrollBehavior get scrollBehavior => super.scrollBehavior;
+  ScrollBehavior createScrollBehavior() => scrollBehavior;
 
   bool get snapScrollOffsetChanges => config.itemsSnapAlignment == ItemsSnapAlignment.item;
 

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

@@ -197,18 +197,6 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   }
 
   Simulation _createFlingSimulation(double velocity) {
-    /*
-    // Assume that we're rendering at atleast 15 FPS. Stop when we're
-    // scrolling less than one logical pixel per frame. We're essentially
-    // normalizing by the devicePixelRatio so that the threshold has the
-    // same effect independent of the device's pixel density.
-    double endVelocity = pixelToScrollOffset(15.0 * ui.window.devicePixelRatio);
-
-    // Similar to endVelocity. Stop scrolling when we're this close to
-    // destiniation scroll offset.
-    double endDistance = pixelToScrollOffset(0.5 * ui.window.devicePixelRatio);
-    */
-
     final double endVelocity = pixelToScrollOffset(kPixelScrollTolerance.velocity);
     final double endDistance = pixelToScrollOffset(kPixelScrollTolerance.distance);
     return scrollBehavior.createFlingScrollSimulation(scrollOffset, velocity)