Breaking change 49647: introduce`exclusive` parameter for File.create, createSync methods

[Alexander Aprelev]

We propose to add new named optional `exclusive` parameter to dart:io `File.create` and `File.createSync` methods. This allows the use of a file system for simple concurrency control.

Code that _implements_ dart:io File class is affected by this change, requires adding this optional parameter to `create` and `createSync` methods.

See discussion on breaking change request: https://github.com/dart-lang/sdk/issues/49647

Please add comments to the issue if you have concerns about this change.