Implement an OverscrollCurve for Scrollable

sky/framework/animation/scroll_curve.dart

Index: sky/framework/animation/scroll_curve.dart
diff --git a/sky/framework/animation/scroll_curve.dart b/sky/framework/animation/scroll_curve.dart
index 07940aee1337040426a9e52e7a36d3f083adc384..54b6385bd1e5d10cccb8fc4dc209ca919b0ab0d9 100644
--- a/sky/framework/animation/scroll_curve.dart
+++ b/sky/framework/animation/scroll_curve.dart
@@ -24,3 +24,18 @@ class BoundedScrollCurve extends ScrollCurve {
     return newScrollOffset;
   }
 }
+
+class OverscrollCurve extends ScrollCurve {
+  double apply(double scrollOffset, double scrollDelta) {
+    double newScrollOffset = scrollOffset + scrollDelta;
+    if (newScrollOffset < 0.0) {
+      // If we're overscrolling, we want move the scroll offset 2x slower than
+      // we would otherwise. Therefore, we "rewind" the newScrollOffset by half
+      // the amount that we moved it above. Notice that we clap the "old" value
+      // to 0.0 so that we only reduce the portion of scrollDelta that's applied
+      // beyond 0.0.
+      newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
+    }
+    return newScrollOffset;
+  }
+}