Fixes https://github.com/creativecreatorormaybenot/performance/issues/18

I used the fix recommended by the flutter team: https://docs.flutter.dev/development/tools/sdk/release-notes/release-notes-3.0.0#your-code

I also run flutter pub get, tell me if you want me to remove those changes

SchedulerBinding.instance is now non-nullable, so the unnecessary_non_null_assertion diagnostic is triggered.

To fix, I ran "dart fix --apply", since this diagnostic can be fixed automatically.

Increases the patch version of this package, since the API isn't changing.

Describe the bug

When running the tests, I get the warning:

../../../.pub-cache/hosted/pub.dartlang.org/performance-0.1.0+1/lib/src/overlay.dart:223:22: Warning: Operand of null-aware operation '!' has type 'SchedulerBinding' which excludes null.
 - 'SchedulerBinding' is from 'package:flutter/src/scheduler/binding.dart' ('../flutter/packages/flutter/lib/src/scheduler/binding.dart').
    SchedulerBinding.instance!.addTimingsCallback(_timingsCallback);
                     ^
../../../.pub-cache/hosted/pub.dartlang.org/performance-0.1.0+1/lib/src/overlay.dart:228:22: Warning: Operand of null-aware operation '!' has type 'SchedulerBinding' which excludes null.
 - 'SchedulerBinding' is from 'package:flutter/src/scheduler/binding.dart' ('../flutter/packages/flutter/lib/src/scheduler/binding.dart').
    SchedulerBinding.instance!.removeTimingsCallback(_timingsCallback);
                     ^
../../../.pub-cache/hosted/pub.dartlang.org/performance-0.1.0+1/lib/src/overlay.dart:251:22: Warning: Operand of null-aware operation '!' has type 'SchedulerBinding' which excludes null.
 - 'SchedulerBinding' is from 'package:flutter/src/scheduler/binding.dart' ('../flutter/packages/flutter/lib/src/scheduler/binding.dart').
    SchedulerBinding.instance!.addPostFrameCallback((timeStamp) {

To Reproduce

I'm using flutter 3.0.5.

Run flutter test on :

// ignore: unused_import
import 'package:performance/performance';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('Counter increments smoke test', (WidgetTester tester) async {});
}

Expected behavior

There should not be any compilation error

Properly cleans up the example app from platform code that is not needed (i.e. non-web because web is needed for the demo app) and explains how to add it back.