Create reference to global app injector with bootstrapped providers

[dustinboss]

Short description of the problem:

Ionic's @app decorator handles the Angular bootstrap process, which is helpful, but it does not store a reference to the global app-injector, which is available after Angular's bootstrap() method resolves. There are cases when this injector is necessary to access the instantiated singleton version of a particular provider. This is sometimes necessary when injecting providers that depend on other providers. This issue is discussed here angular/angular#4112 (comment), and Brandon Roberts' solution of an appInjector() method has been used to solve a variety of dependency injection conflicts.

What behavior are you expecting?

Be able to reference the Angular Injector reference that is returned from the bootstrap() method. I have provided PR #5974 to facilitate this via the IonicApp provider

let appInjector = IonicApp.getAppInjector();

Which Ionic Version? 1.x or 2.x
2.0

Run ionic info from terminal/cmd prompt: (paste output below)
Cordova CLI: Not installed
Gulp version: CLI version 1.2.1
Gulp local: Local version 3.8.11
Ionic CLI Version: 2.0.0-beta.19
Ionic App Lib Version: 2.0.0-beta.9
ios-deploy version: Not installed
ios-sim version: Not installed
OS: Mac OS X El Capitan
Node Version: v4.2.0
Xcode version: Xcode 7.2.1 Build version 7C1002