ScrollableList was applying padding twice

We need to subtract the padding instead of adding it.

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

@@ -193,8 +193,8 @@ class _VirtualListViewportElement extends VirtualViewportElement<_VirtualListVie
       _startOffsetBase = 0.0;
       _startOffsetLimit = double.INFINITY;
     } else {
-      int startItem = math.max(0, (widget.startOffset + leadingPadding) ~/ itemExtent);
-      int limitItem = math.max(0, ((widget.startOffset + leadingPadding + containerExtent) / itemExtent).ceil());
+      int startItem = math.max(0, (widget.startOffset - leadingPadding) ~/ itemExtent);
+      int limitItem = math.max(0, ((widget.startOffset - leadingPadding + containerExtent) / itemExtent).ceil());
 
       if (!widget.itemsWrap && length != null) {
         startItem = math.min(length, startItem);

packages/flutter/test/widget/scrollable_list_vertical_test.dart

@@ -65,4 +65,45 @@ void main() {
       expect(tester.findText('5'), isNull);
     });
   });
+
+  test('Drag vertically', () {
+    testWidgets((WidgetTester tester) {
+      tester.pumpWidget(
+        new ScrollableList(
+          itemExtent: 290.0,
+          padding: new EdgeDims.only(top: 250.0),
+          scrollDirection: Axis.vertical,
+          children: items.map((int item) {
+            return new Container(
+              child: new Text('$item')
+            );
+          })
+        )
+      );
+
+      tester.pump();
+      // screen is 600px high, and has the following items:
+      //   250..540 = 0
+      //   540..830 = 1
+      expect(tester.findText('0'), isNotNull);
+      expect(tester.findText('1'), isNotNull);
+      expect(tester.findText('2'), isNull);
+      expect(tester.findText('3'), isNull);
+      expect(tester.findText('4'), isNull);
+      expect(tester.findText('5'), isNull);
+
+      tester.scroll(tester.findText('0'), const Offset(0.0, -300.0));
+      tester.pump();
+      // screen is 600px high, and has the following items:
+      //   -50..240 = 0
+      //   240..530 = 1
+      //   530..820 = 2
+      expect(tester.findText('0'), isNotNull);
+      expect(tester.findText('1'), isNotNull);
+      expect(tester.findText('2'), isNotNull);
+      expect(tester.findText('3'), isNull);
+      expect(tester.findText('4'), isNull);
+      expect(tester.findText('5'), isNull);
+    });
+  });
 }