Handle dragging a bottom sheet past the bottom (#5541)

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -108,11 +108,13 @@ class _BottomSheetState extends State<BottomSheet> {
       return;
     if (details.velocity.pixelsPerSecond.dy > _kMinFlingVelocity) {
       double flingVelocity = -details.velocity.pixelsPerSecond.dy / _childHeight;
-      config.animationController.fling(velocity: flingVelocity);
+      if (config.animationController.value > 0.0)
+        config.animationController.fling(velocity: flingVelocity);
       if (flingVelocity < 0.0)
         config.onClosing();
     } else if (config.animationController.value < _kCloseProgressThreshold) {
-      config.animationController.fling(velocity: -1.0);
+      if (config.animationController.value > 0.0)
+        config.animationController.fling(velocity: -1.0);
       config.onClosing();
     } else {
       config.animationController.forward();

packages/flutter/lib/src/material/scaffold.dart

@@ -454,7 +454,7 @@ class ScaffoldState extends State<Scaffold> {
 
   // PERSISTENT BOTTOM SHEET API
 
-  List<Widget> _dismissedBottomSheets;
+  final List<Widget> _dismissedBottomSheets = <Widget>[];
   PersistentBottomSheetController<dynamic> _currentBottomSheet;
 
   /// Shows a persistent material design bottom sheet.
@@ -491,9 +491,11 @@ class ScaffoldState extends State<Scaffold> {
         assert(_currentBottomSheet._widget == bottomSheet);
         assert(bottomSheetKey.currentState != null);
         bottomSheetKey.currentState.close();
-        _dismissedBottomSheets ??= <Widget>[];
-        _dismissedBottomSheets.add(bottomSheet);
-        _currentBottomSheet = null;
+        if (controller.status != AnimationStatus.dismissed)
+          _dismissedBottomSheets.add(bottomSheet);
+        setState(() {
+          _currentBottomSheet = null;
+        });
         completer.complete();
       }
     );
@@ -505,10 +507,11 @@ class ScaffoldState extends State<Scaffold> {
         entry.remove();
       },
       onDismissed: () {
-        assert(_dismissedBottomSheets != null);
-        setState(() {
-          _dismissedBottomSheets.remove(bottomSheet);
-        });
+        if (_dismissedBottomSheets.contains(bottomSheet)) {
+          setState(() {
+            _dismissedBottomSheets.remove(bottomSheet);
+          });
+        }
       },
       builder: builder
     );
@@ -728,10 +731,9 @@ class ScaffoldState extends State<Scaffold> {
     }
     // Otherwise the AppBar will be part of a [app bar, body] Stack. See AppBarBehavior.scroll below.
 
-    if (_currentBottomSheet != null ||
-        (_dismissedBottomSheets != null && _dismissedBottomSheets.isNotEmpty)) {
+    if (_currentBottomSheet != null || _dismissedBottomSheets.isNotEmpty) {
       final List<Widget> bottomSheets = <Widget>[];
-      if (_dismissedBottomSheets != null && _dismissedBottomSheets.isNotEmpty)
+      if (_dismissedBottomSheets.isNotEmpty)
         bottomSheets.addAll(_dismissedBottomSheets);
       if (_currentBottomSheet != null)
         bottomSheets.add(_currentBottomSheet._widget);

packages/flutter/test/widget/bottom_sheet_test.dart

@@ -110,4 +110,33 @@ void main() {
     expect(find.text('BottomSheet'), findsNothing);
   });
 
+  testWidgets('Verify that dragging past the bottom dismisses a persistent BottomSheet', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/5528
+    GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();
+
+    await tester.pumpWidget(new MaterialApp(
+      home: new Scaffold(
+        key: scaffoldKey,
+        body: new Center(child: new Text('body'))
+      )
+    ));
+
+    scaffoldKey.currentState.showBottomSheet((BuildContext context) {
+      return new Container(
+        margin: new EdgeInsets.all(40.0),
+        child: new Text('BottomSheet')
+      );
+    });
+
+    await tester.pump(); // bottom sheet show animation starts
+    await tester.pump(new Duration(seconds: 1)); // animation done
+    expect(find.text('BottomSheet'), findsOneWidget);
+
+    await tester.fling(find.text('BottomSheet'), const Offset(0.0, 400.0), 1000.0);
+    await tester.pump(); // drain the microtask queue (Future completion callback)
+    await tester.pump(); // bottom sheet dismiss animation starts
+    await tester.pump(new Duration(seconds: 1)); // animation done
+
+    expect(find.text('BottomSheet'), findsNothing);
+  });
 }