added i18n tests; added top-level refs to app,filesystem, i18n, and stor...

added i18n tests; added top-level refs to app,filesystem, i18n, and storage

This adds references to the app.dart, file_system, storage, and i18n.dart code from chrome.dart. It also adds top-level variables in chrome.dart so importers can do this:

import 'package:chrome/chrome.dart' as chrome;

and then write their code like this

chrome.app.window.current()

which will look very similar to the JavaScript usage. Let me know what you think!

@financeCoding