Support flutter 3.0

[hs029]

See this error:
../../../appdev/flutter/.pub-cache/hosted/pub.flutter-io.cn/fsuper_nullsafety-3.0.1/lib/fsuper_nullsafety.dart:330:20: Warning: Operand of null-aware operation '!' has type 'WidgetsBinding' which excludes null.

• 'WidgetsBinding' is from 'package:flutter/src/widgets/binding.dart' ('../../../appdev/flutter/packages/flutter/lib/src/widgets/binding.dart').
  WidgetsBinding.instance!.addPostFrameCallback(_handleSizeChanged);
  ^
  ../../../appdev/flutter/.pub-cache/hosted/pub.flutter-io.cn/fsuper_nullsafety-3.0.1/lib/fsuper_nullsafety.dart:342:20: Warning: Operand of null-aware operation '!' has type 'WidgetsBinding' which excludes null.
• 'WidgetsBinding' is from 'package:flutter/src/widgets/binding.dart' ('../../../appdev/flutter/packages/flutter/lib/src/widgets/binding.dart').
  WidgetsBinding.instance!.addPostFrameCallback(_handleSizeChanged);
  ^
  ../../../appdev/flutter/.pub-cache/hosted/pub.flutter-io.cn/fsuper_nullsafety-3.0.1/lib/fsuper_nullsafety.dart:626:22: Warning: Operand of null-aware operation '!' has type 'SchedulerBinding' which excludes null.
• 'SchedulerBinding' is from 'package:flutter/src/scheduler/binding.dart' ('../../../appdev/flutter/packages/flutter/lib/src/scheduler/binding.dart').
  SchedulerBinding.instance!.addPostFrameCallback(postFrameCallback);
  ^

[mdddj]

fsuper_nullsafety: ^3.0.3