Often some init logic needs to run when a platform or an application is boostrapped.
For example, boostraping a platform requires initializing the dom adapter.
Now, it can be done as follows:
new Provider(PLATFORM_INITIALIZER, {useValue: initDomAdapter, multi: true}),
All platform initializers will be run after the platform injector has been created.
Similarly, all application initializers will be run after the app injector has been
created.
Closes#5355
3c43a8c549