Make use of field promotion in SDK libraries

[stereotype441]

https://dart-review.googlesource.com/c/sdk/+/314500 refactors several places in the SDK to avoid having the build fail due to "unnecessary !" warnings when we enable the language feature inference-update-2 (which provides support for promotion of final private fields). For example, it changes:

Directory createDirectory(String path) {
  if (_createDirectory != null) return _createDirectory!(path);
  if (_previous != null) return _previous!.createDirectory(path);
  return super.createDirectory(path);
}

to

Directory createDirectory(String path) {
  if (_createDirectory != null) return _createDirectory!(path);
  var previous = _previous;
  if (previous != null) return previous.createDirectory(path);
  return super.createDirectory(path);
}

After interface-update-2 has been enabled, we should consider refactoring this to:

Directory createDirectory(String path) {
  if (_createDirectory != null) return _createDirectory!(path);
  if (_previous != null) return _previous.createDirectory(path);
  return super.createDirectory(path);
}

(note the absence of the ! in _previous.createDirectory).