toString is a dangerous name for the NSString->String conversion method

[stuartmorgan]

Spot the bug in the code below:

  @override
  Future<String?> getLibraryPath() async {
    return _getUserDirectory(NSSearchPathDirectory.NSLibraryDirectory).toString();
  }

  @override
  Future<String?> getApplicationDocumentsPath() async {
    return _getUserDirectory(NSSearchPathDirectory.NSDocumentDirectory).toString();
  }

  [...]

  NSString? _getUserDirectory(NSSearchPathDirectory directory) {
    ...
  }

I would strongly prefer a toDartString method to do the conversion so that this code (using toDartString in place of toString) wouldn't compile. The current version seems extremely likely to cause mistakes like the above in actual usage, and they won't be caught until runtime.