Migrate missed sample code to NNBD (#73509)

packages/flutter/lib/foundation.dart

@@ -19,21 +19,6 @@ export 'package:meta/meta.dart' show
   required,
   visibleForTesting;
 
-// Examples can assume:
-// // @dart = 2.9
-// String _name;
-// bool _first;
-// bool _lights;
-// bool _visible;
-// bool inherit;
-// int columns;
-// int rows;
-// class Cat { }
-// double _volume;
-// dynamic _calculation;
-// dynamic _last;
-// dynamic _selection;
-
 export 'src/foundation/annotations.dart';
 export 'src/foundation/assertions.dart';
 export 'src/foundation/basic_types.dart';

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

@@ -21,11 +21,6 @@ import 'theme.dart';
 // CalendarDatePicker (if needed, as showDatePicker has already been migrated
 // and it is what most apps would have used).
 
-
-// Examples can assume:
-// // @dart = 2.9
-// BuildContext context;
-
 const Duration _kMonthScrollDuration = Duration(milliseconds: 200);
 const double _kDayPickerRowHeight = 42.0;
 const int _kMaxDayPickerRowCount = 6; // A 31 day month that starts on Saturday.

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

@@ -22,10 +22,6 @@ import 'scroll_view.dart';
 import 'sliver_fill.dart';
 import 'viewport.dart';
 
-// Examples can assume:
-// // @dart = 2.9
-// List<String> _tabs;
-
 /// Signature used by [NestedScrollView] for building its header.
 ///
 /// The `innerBoxIsScrolled` argument is typically used to control the

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

@@ -275,8 +275,7 @@ class ScrollController extends ChangeNotifier {
 }
 
 // Examples can assume:
-// // @dart = 2.9
-// TrackingScrollController _trackingScrollController;
+// TrackingScrollController? _trackingScrollController;
 
 /// A [ScrollController] whose [initialScrollOffset] tracks its most recently
 /// updated [ScrollPosition].