Merge pull request #2465 from Hixie/cleanup

Cleanup around viewports

packages/flutter/lib/src/rendering/viewport.dart

@@ -195,6 +195,14 @@ class RenderViewportBase extends RenderBox implements HasScrollDirection {
     super.applyPaintTransform(child, transform.translate(effectivePaintOffset.dx, effectivePaintOffset.dy));
   }
 
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('paintOffset: $paintOffset');
+    description.add('scrollDirection: $scrollDirection');
+    description.add('scrollAnchor: $scrollAnchor');
+    if (overlayPainter != null)
+      description.add('overlay painter: $overlayPainter');
+  }
 }
 
 typedef Offset ViewportDimensionsChangeCallback(ViewportDimensions dimensions);
@@ -373,4 +381,9 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
   }
 
   Rect describeApproximatePaintClip(RenderObject child) => Point.origin & size;
+
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('virtual child count: $virtualChildCount');
+  }
 }

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

@@ -71,6 +71,7 @@ abstract class ExtentScrollBehavior extends ScrollBehavior<double, double> {
     double containerExtent,
     double scrollOffset: 0.0
   }) {
+    assert(minScrollOffset <= maxScrollOffset);
     if (contentExtent != null)
       _contentExtent = contentExtent;
     if (containerExtent != null)
@@ -109,8 +110,10 @@ class BoundedBehavior extends ExtentScrollBehavior {
     double minScrollOffset,
     double scrollOffset: 0.0
   }) {
-    if (minScrollOffset != null)
+    if (minScrollOffset != null) {
       _minScrollOffset = minScrollOffset;
+      assert(minScrollOffset <= maxScrollOffset);
+    }
     return super.updateExtents(
       contentExtent: contentExtent,
       containerExtent: containerExtent,