ItemBuilder should also get the index of the item

This seems useful for numbering the entries in the list, for example.

Fixes #328

examples/fitness/lib/feed.dart

@@ -18,7 +18,7 @@ class FitnessItemList extends StatelessComponent {
       padding: const EdgeDims.all(4.0),
       items: items,
       itemExtent: kFitnessItemHeight,
-      itemBuilder: (BuildContext context, FitnessItem item) => item.toRow(onDismissed: onDismissed)
+      itemBuilder: (BuildContext context, FitnessItem item, int index) => item.toRow(onDismissed: onDismissed)
     );
   }
 }

examples/stocks/lib/stock_list.dart

@@ -16,7 +16,7 @@ class StockList extends StatelessComponent {
     return new ScrollableList<Stock>(
       items: stocks,
       itemExtent: StockRow.kHeight,
-      itemBuilder: (BuildContext context, Stock stock) {
+      itemBuilder: (BuildContext context, Stock stock, int index) {
         return new StockRow(
           stock: stock,
           onPressed: onOpen,

examples/widgets/card_collection.dart

@@ -400,7 +400,7 @@ class CardCollectionState extends State<CardCollection> {
         snapOffsetCallback: _snapToCenter ? _toSnapOffset : null,
         snapAlignmentOffset: _cardCollectionSize.height / 2.0,
         items: _cardModels,
-        itemBuilder: (BuildContext context, CardModel card) => buildCard(context, card.value),
+        itemBuilder: (BuildContext context, CardModel card, int index) => buildCard(context, card.value),
         itemExtent: _cardModels[0].height
       );
     } else {

examples/widgets/pageable_list.dart

@@ -47,7 +47,7 @@ class PageableListAppState extends State<PageableListApp> {
     });
   }
 
-  Widget buildCard(BuildContext context, CardModel cardModel) {
+  Widget buildCard(BuildContext context, CardModel cardModel, int index) {
     Widget card = new Card(
       color: cardModel.color,
       child: new Container(

examples/widgets/scrollbar.dart

@@ -19,9 +19,9 @@ class ScrollbarAppState extends State<ScrollbarApp> {
     return new ScrollableList<int>(
       items: new List<int>.generate(_itemCount, (int i) => i),
       itemExtent: _itemExtent,
-      itemBuilder: (_, int i) {
-        return new Text('Item ${dd.format(i)}',
-          key: new ValueKey<int>(i),
+      itemBuilder: (_, __, int index) {
+        return new Text('Item ${dd.format(index)}',
+          key: new ValueKey<int>(index),
           style: Theme.of(context).text.title
         );
       },

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

@@ -632,7 +632,7 @@ abstract class ScrollableWidgetListState<T extends ScrollableWidgetList> extends
 
 }
 
-typedef Widget ItemBuilder<T>(BuildContext context, T item);
+typedef Widget ItemBuilder<T>(BuildContext context, T item, int index);
 
 /// A wrapper around [ScrollableWidgetList] that helps you translate a list of
 /// model objects into a scrollable list of widgets. Assumes all the widgets
@@ -682,7 +682,7 @@ class ScrollableListState<T, Config extends ScrollableList<T>> extends Scrollabl
     int begin = config.itemsWrap ? start : math.max(0, start);
     int end = config.itemsWrap ? begin + count : math.min(begin + count, config.items.length);
     for (int i = begin; i < end; ++i)
-      result.add(config.itemBuilder(context, config.items[i % itemCount]));
+      result.add(config.itemBuilder(context, config.items[i % itemCount], i));
     return result;
   }
 }

packages/unit/test/widget/dismissable_test.dart

@@ -19,7 +19,7 @@ void handleOnDismissed(item) {
   dismissedItems.add(item);
 }
 
-Widget buildDismissableItem(BuildContext context, int item) {
+Widget buildDismissableItem(BuildContext context, int item, int index) {
   return new Dismissable(
     key: new ValueKey<int>(item),
     direction: dismissDirection,

packages/unit/test/widget/pageable_list_test.dart

@@ -8,7 +8,7 @@ const List<int> pages = const <int>[0, 1, 2, 3, 4, 5];
 int currentPage = null;
 bool itemsWrap = false;
 
-Widget buildPage(BuildContext context, int page) {
+Widget buildPage(BuildContext context, int page, int index) {
   return new Container(
     key: new ValueKey<int>(page),
     width: pageSize.width,

packages/unit/test/widget/reparent_state_test.dart

@@ -103,7 +103,7 @@ void main() {
       tester.pumpWidget(new ScrollableList<int>(
         items: <int>[0],
         itemExtent: 100.0,
-        itemBuilder: (BuildContext context, int item) {
+        itemBuilder: (BuildContext context, int item, int index) {
           return new Container(
             key: new Key('container'),
             height: 100.0,

packages/unit/test/widget/scrollable_list_hit_testing_test.dart

@@ -17,7 +17,7 @@ void main() {
           child: new ScrollableList<int>(
             key: new GlobalKey(),
             items: items,
-            itemBuilder: (BuildContext context, int item) {
+            itemBuilder: (BuildContext context, int item, int index) {
               return new Container(
                 key: new ValueKey<int>(item),
                 child: new GestureDetector(
@@ -59,7 +59,7 @@ void main() {
           child: new ScrollableList<int>(
             key: new GlobalKey(),
             items: items,
-            itemBuilder: (BuildContext context, int item) {
+            itemBuilder: (BuildContext context, int item, int index) {
               return new Container(
                 key: new ValueKey<int>(item),
                 child: new GestureDetector(

packages/unit/test/widget/scrollable_list_horizontal_test.dart

@@ -12,7 +12,7 @@ Widget buildFrame() {
       height: 50.0,
       child: new ScrollableList<int>(
         items: items,
-        itemBuilder: (BuildContext context, int item) {
+        itemBuilder: (BuildContext context, int item, int index) {
           return new Container(
             key: new ValueKey<int>(item),
             child: new Text('$item')

packages/unit/test/widget/scrollable_list_vertical_test.dart

@@ -8,7 +8,7 @@ const List<int> items = const <int>[0, 1, 2, 3, 4, 5];
 Widget buildFrame() {
   return new ScrollableList<int>(
     items: items,
-    itemBuilder: (BuildContext context, int item) {
+    itemBuilder: (BuildContext context, int item, int index) {
       return new Container(
         key: new ValueKey<int>(item),
         child: new Text('$item')

packages/unit/test/widget/snap_scrolling_test.dart

@@ -13,7 +13,7 @@ const double itemExtent = 200.0;
 ScrollDirection scrollDirection = ScrollDirection.vertical;
 GlobalKey scrollableListKey;
 
-Widget buildItem(BuildContext context, int item) {
+Widget buildItem(BuildContext context, int item, int index) {
   return new Container(
     key: new ValueKey<int>(item),
     width: itemExtent,