Convert dismissable_test.dart to ListView (#8078)

This test was a bit tricky to convert because it subtly relied upon the
lazy evaluation of an Iterable.

The onDismissed from Dismissable happens during the animation phase of
the pipeline. Previously, the ScrollableList had already been built for
that frame but had not evaluated its Iterable yet. When we got to the
layout phase, ScrollableList evaluated its Iterable and saw the updated
version of dismissedItems.

A straightforward conversion to ListView calls toList() when building
the ListView, but that evaluates the iterable when buildTest() is
called, which is before the calls to pump and therefore before the
animation phase, meaning the Iterable sees the old value of
dismissedItems.

This patch fixes the test to use the normal setState pattern to signal
that state upon which the build depends has changed. Now, the
onDismissed callback happens during the animation phase and the
StatefulBuilder is marked as dirty via setState, which causes it to
rebuild the ListView and re-evaluate the Iterable, seeing the updated
version of dismissedItems.

This change also lets us replace the gratuious use of pumpWidget with
pump now that we use setState rather than pumpWidget to trigger a
rebuild.

packages/flutter/test/widgets/dismissable_test.dart

@@ -13,26 +13,22 @@ DismissDirection reportedDismissDirection;
 List<int> dismissedItems = <int>[];
 Widget background;
 
-void handleOnResize(int item) {
-  expect(dismissedItems.contains(item), isFalse);
-}
-
-void handleOnDismissed(DismissDirection direction, int item) {
-  reportedDismissDirection = direction;
-  expect(dismissedItems.contains(item), isFalse);
-  dismissedItems.add(item);
-}
-
 Widget buildTest({ double startToEndThreshold }) {
+  return new StatefulBuilder(
+    builder: (BuildContext context, StateSetter setState) {
       Widget buildDismissableItem(int item) {
         return new Dismissable(
           key: new ValueKey<int>(item),
           direction: dismissDirection,
           onDismissed: (DismissDirection direction) {
-        handleOnDismissed(direction, item);
+            setState(() {
+              reportedDismissDirection = direction;
+              expect(dismissedItems.contains(item), isFalse);
+              dismissedItems.add(item);
+            });
           },
           onResize: () {
-        handleOnResize(item);
+            expect(dismissedItems.contains(item), isFalse);
           },
           background: background,
           dismissThresholds: startToEndThreshold == null
@@ -41,20 +37,22 @@ Widget buildTest({ double startToEndThreshold }) {
           child: new Container(
             width: itemExtent,
             height: itemExtent,
-        child: new Text(item.toString())
+            child: new Text(item.toString()),
           ),
         );
       }
 
       return new Container(
         padding: const EdgeInsets.all(10.0),
-      child: new ScrollableList(
+        child: new ListView(
           scrollDirection: scrollDirection,
           itemExtent: itemExtent,
           children: <int>[0, 1, 2, 3, 4]
             .where((int i) => !dismissedItems.contains(i))
-          .map(buildDismissableItem),
-      )
+            .map(buildDismissableItem).toList(),
+        ),
+      );
+    },
   );
 }
 
@@ -106,11 +104,11 @@ Future<Null> dismissItem(WidgetTester tester, int item, { DismissDirection gestu
 
   await dismissElement(tester, itemFinder, gestureDirection: gestureDirection);
 
-  await tester.pumpWidget(buildTest()); // start the slide
-  await tester.pumpWidget(buildTest(), const Duration(seconds: 1)); // finish the slide and start shrinking...
-  await tester.pumpWidget(buildTest()); // first frame of shrinking animation
-  await tester.pumpWidget(buildTest(), const Duration(seconds: 1)); // finish the shrinking and call the callback...
-  await tester.pumpWidget(buildTest()); // rebuild after the callback removes the entry
+  await tester.pump(); // start the slide
+  await tester.pump(const Duration(seconds: 1)); // finish the slide and start shrinking...
+  await tester.pump(); // first frame of shrinking animation
+  await tester.pump(const Duration(seconds: 1)); // finish the shrinking and call the callback...
+  await tester.pump(); // rebuild after the callback removes the entry
 }
 
 class Test1215DismissableWidget extends StatelessWidget {