Fix padding on infinite scrolling list

Rather than using a Padding widget to provide padding along the scrolling axis,
we now just figure the padding into where we draw the items. This patch fixes
an issue where we would remove the first topmost item in a scrollable list too
early because we thought it was already off screen.

Fixes #697

packages/flutter/lib/widgets/scrollable.dart

@@ -384,14 +384,30 @@ abstract class FixedHeightScrollable extends Scrollable {
     });
   }
 
-  void _updateContentsExtent() {
-    double contentsExtent = itemExtent * itemCount;
-    if (padding != null) {
+  double get _leadingPadding {
+    if (scrollDirection == ScrollDirection.vertical)
+      return padding.top;
+    return padding.left;
+  }
+
+  double get _trailingPadding {
+    if (scrollDirection == ScrollDirection.vertical)
+      return padding.bottom;
+    return padding.right;
+  }
+
+  EdgeDims get _crossAxisPadding {
+    if (padding == null)
+      return null;
     if (scrollDirection == ScrollDirection.vertical)
-        contentsExtent += padding.top + padding.bottom;
-      else
-        contentsExtent += padding.left + padding.right;
+      return new EdgeDims.only(left: padding.left, right: padding.right);
+    return new EdgeDims.only(top: padding.top, bottom: padding.bottom);
   }
+
+  void _updateContentsExtent() {
+    double contentsExtent = itemExtent * itemCount;
+    if (padding != null)
+      contentsExtent += _leadingPadding + _trailingPadding;
     scrollBehavior.contentsSize = contentsExtent;
   }
 
@@ -413,25 +429,29 @@ abstract class FixedHeightScrollable extends Scrollable {
       _updateScrollOffset();
     }
 
+    double paddedScrollOffset = scrollOffset;
+    if (padding != null)
+      paddedScrollOffset -= _leadingPadding;
+
     int itemShowIndex = 0;
     int itemShowCount = 0;
     Offset viewportOffset = Offset.zero;
     if (_containerExtent != null && _containerExtent > 0.0) {
-      if (scrollOffset < 0.0) {
-        double visibleHeight = _containerExtent + scrollOffset;
+      if (paddedScrollOffset < 0.0) {
+        double visibleHeight = _containerExtent + paddedScrollOffset;
         itemShowCount = (visibleHeight / itemExtent).round() + 1;
-        viewportOffset = _toOffset(scrollOffset);
+        viewportOffset = _toOffset(paddedScrollOffset);
       } else {
         itemShowCount = (_containerExtent / itemExtent).ceil();
-        double alignmentDelta = -scrollOffset % itemExtent;
+        double alignmentDelta = -paddedScrollOffset % itemExtent;
         double drawStart;
         if (alignmentDelta != 0.0) {
           alignmentDelta -= itemExtent;
           itemShowCount += 1;
-          drawStart = scrollOffset + alignmentDelta;
+          drawStart = paddedScrollOffset + alignmentDelta;
           viewportOffset = _toOffset(-alignmentDelta);
         } else {
-          drawStart = scrollOffset;
+          drawStart = paddedScrollOffset;
         }
         itemShowIndex = math.max(0, (drawStart / itemExtent).floor());
       }
@@ -453,7 +473,7 @@ abstract class FixedHeightScrollable extends Scrollable {
         scrollDirection: scrollDirection,
         scrollOffset: viewportOffset,
         child: new Container(
-          padding: padding,
+          padding: _crossAxisPadding,
           child: new Block(items, direction: blockDirection)
         )
       )