Flutter run not working with null-safety flag

**Describe the bug**
More recent version of flutter implement the null safety feature, which throws out errors if not run with the `--no-sound-null-safety` flag. When running flutter from the terminal this works as normal (even without the flag), however when using the inbuilt `CocCommand flutter.run --no-sound-null-safety` there are errors which indicate that this flag isnt parsed.

There is only 1 version of flutter on my system (which I reinstalled using the stable channel).
```
Flutter 2.8.1 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 77d935af4d (11 days ago) • 2021-12-16 08:37:33 -0800
Engine • revision 890a5fca2e
Tools • Dart 2.15.1
```

```
Error: Cannot run with sound null safety, because the following dependencies don't support null safety:
.....
lib/main.dart:230:23: Error: The parameter 'controller' can't have a value of 'null' because of its type 'TextEditingController', but the implicit default value is 'null'.
```

**To Reproduce**
Steps to reproduce the behavior:

1. run :CocCommand flutter.run --no-sound-null-safety
2. it doesn't say anywhere that it's using the flag

**Expected behavior**
I expect it to use the flag and work just like the normal terminal command.

**Desktop (please complete the following information):**
- OS: Windows 11
- Vim or Neovim: Neovim
- (Neo)vim version: NVIM v0.6.0

This is copy of https://github.com/iamcco/coc-flutter/issues/78 as it was noted there that a new ticket should be opened for this issue