IOOverrides class Null safety

Facilities for overriding various APIs of dart:io with mock implementations.

This abstract base class should be extended with overrides for the operations needed to construct mocks. The implementations in this base class default to the actual dart:io implementation. For example:

class MyDirectory implements Directory {
  ...
  // An implementation of the Directory interface
  ...
}

void main() {
  IOOverrides.runZoned(() {
    ...
    // Operations will use MyDirectory instead of dart:io's Directory
    // implementation whenever Directory is used.
    ...
  }, createDirectory: (String path) => new MyDirectory(path));
}