Make method createGitHubClient support vm flutter and web.

[CaiJingLong]

Imitate the way http is written, let the createGitHubClient method work on dart vm, flutter and web.

[CaiJingLong]

This way of import comes from the http library.

Now whether it is vm or web or flutter, we can use the same code to create github client.
Just like this:

import 'package:github/github.dart';

void main(){
  final auth = Authentication.basic("", "");

  final github = createGitHubClient(
    auth: auth,
  );

  final user = await github.users.getCurrentUser();
  print(user.toJson());

  github.dispose();
}

[robrbecker]

Interesting! I'll take a look. Thanks!

[robrbecker]

After reviewing, I think this will work in the short term. I'll get this merged and come back to further clean up the split between dart:html and dart:io. If using > 0.12.0 of the http package, it gets a lot easier.