Fix missing return statements on function literals (#33058)

examples/flutter_gallery/lib/demo/material/leave_behind_demo.dart

@@ -243,6 +243,7 @@ class _LeaveBehindListItem extends StatelessWidget {
             case DismissDirection.down:
               assert(false);
           }
+          return true;
         },
         background: Container(
           color: theme.primaryColor,

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -2250,4 +2250,5 @@ final HeroFlightShuttleBuilder _navBarHeroFlightShuttleBuilder = (
         topNavBar: fromNavBar,
       );
   }
+  return null;
 };

packages/flutter_driver/lib/src/driver/driver.dart

@@ -101,7 +101,7 @@ Future<T> _warnIfSlow<T>({
   assert(future != null);
   assert(timeout != null);
   assert(message != null);
-  return future..timeout(timeout, onTimeout: () { _log.warning(message); });
+  return future..timeout(timeout, onTimeout: () { _log.warning(message); return null; });
 }
 
 /// A convenient accessor to frequently used finders.

packages/flutter_test/lib/src/binding.dart

@@ -764,6 +764,7 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
           final Uint8List encoded = Uint8List.fromList(asset.readAsBytesSync());
           return Future<ByteData>.value(encoded.buffer.asByteData());
         }
+        return null;
       });
     }
   }

packages/flutter_test/test/window_test.dart

@@ -164,7 +164,7 @@ void main() {
     );
   });
 
-  testWidgets('TestWindow can clear out fake properties all at once', (WidgetTester tester) {
+  testWidgets('TestWindow can clear out fake properties all at once', (WidgetTester tester) async {
     final double originalDevicePixelRatio = ui.window.devicePixelRatio;
     final double originalTextScaleFactor = ui.window.textScaleFactor;
     final TestWindow testWindow = retrieveTestBinding(tester).window;

packages/flutter_tools/test/src/testbed.dart

@@ -79,6 +79,7 @@ class Testbed {
           }
           await test();
           Cache.flutterRoot = originalFlutterRoot;
+          return null;
         }
       );
     });

packages/flutter_tools/test/test_compiler_test.dart

@@ -19,7 +19,7 @@ void main() {
 
     setUp(() {
       testbed = Testbed(
-        setup: () {
+        setup: () async {
           fs.file('pubspec.yaml').createSync();
           fs.file('.packages').createSync();
           fs.file('test/foo.dart').createSync(recursive: true);