Notify the completer after the close animation completes on the snackbar (#11688)

* Notify the completer after the close animation completes on the snackbar

* Review comments

* Fix tests and analyzer warnings

* Fix analyzer warnings

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

@@ -605,14 +605,18 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   }
 
   /// Removes the current [SnackBar] by running its normal exit animation.
+  ///
+  /// The closed completer is called after the animation is complete.
   void hideCurrentSnackBar({ SnackBarClosedReason reason: SnackBarClosedReason.hide }) {
     assert(reason != null);
     if (_snackBars.isEmpty || _snackBarController.status == AnimationStatus.dismissed)
       return;
     final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
-    if (!completer.isCompleted)
-      completer.complete(reason);
-    _snackBarController.reverse();
+    _snackBarController.reverse().then<Null>((Null _) {
+      assert(mounted);
+      if (!completer.isCompleted)
+        completer.complete(reason);
+    });
     _snackBarTimer?.cancel();
     _snackBarTimer = null;
   }

packages/flutter/test/material/snack_bar_test.dart

@@ -374,7 +374,11 @@ void main() {
     expect(actionPressed, isFalse);
     await tester.tap(find.text('ACTION'));
     expect(actionPressed, isTrue);
-    await tester.pump(const Duration(seconds: 1));
+    // Closed reason is only set when the animation is complete.
+    await tester.pump(const Duration(milliseconds:250));
+    expect(closedReason, isNull);
+    // Wait for animation to complete.
+    await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.action));
 
     // Pop up the snack bar and then swipe downwards to dismiss it.
@@ -382,21 +386,21 @@ void main() {
     await tester.pump(const Duration(milliseconds: 750));
     await tester.pump(const Duration(milliseconds: 750));
     await tester.drag(find.text('snack'), const Offset(0.0, 50.0));
-    await tester.pump();
+    await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.swipe));
 
     // Pop up the snack bar and then remove it.
     await tester.tap(find.text('X'));
     await tester.pump(const Duration(milliseconds: 750));
     scaffoldKey.currentState.removeCurrentSnackBar();
-    await tester.pump(const Duration(seconds: 1));
+    await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.remove));
 
     // Pop up the snack bar and then hide it.
     await tester.tap(find.text('X'));
     await tester.pump(const Duration(milliseconds: 750));
     scaffoldKey.currentState.hideCurrentSnackBar();
-    await tester.pump(const Duration(seconds: 1));
+    await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.hide));
 
     // Pop up the snack bar and then let it time out.
@@ -405,7 +409,7 @@ void main() {
     await tester.pump(const Duration(milliseconds: 750));
     await tester.pump(const Duration(milliseconds: 1500));
     await tester.pump(); // begin animation
-    await tester.pump(const Duration(milliseconds: 750));
+    await tester.pumpAndSettle(const Duration(seconds: 1));
     expect(closedReason, equals(SnackBarClosedReason.timeout));
   });