OMG I've been running with checking turned off. Fixed scrollbar assertion failures.

examples/widgets/scrollbar.dart

@@ -23,7 +23,12 @@ class ScrollbarAppState extends State<ScrollbarApp> {
     return new ScrollableList<int>(
       items: new List<int>.generate(_itemCount, (int i) => i),
       itemExtent: _itemExtent,
-      itemBuilder: (BuildContext _, int i) => new Text('Item ${dd.format(i)}', style: Theme.of(context).text.title),
+      itemBuilder: (_, int i) {
+        return new Text('Item ${dd.format(i)}',
+          key: new ValueKey<int>(i),
+          style: Theme.of(context).text.title
+        );
+      },
       scrollableListPainter: _scrollbarPainter
     );
   }

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

@@ -434,7 +434,6 @@ abstract class ScrollableListPainter extends Painter {
   double _scrollOffset = 0.0;
   void set scrollOffset (double value) {
     assert(value != null);
-    assert(value >= 0.0 && value <= 1.0);
     if (_scrollOffset == value)
       return;
     _scrollOffset = value;