Ensure Singleton Provider (AOT compatible)

utils.ts

export function singletonFactory<T>(token: InjectionToken<T>, factory: Function, deps: any[], fnToken = new InjectionToken('')) {
	return [
		{
			provide: fnToken,
			useValue: factory
		},
		{
			provide: token,
			deps: [[new SkipSelf(), new Optional(), token], fnToken, ...(deps ? deps : [])],
			useFactory: function(parent: T | null, factoryFn: Function, ...args) {
				if (parent != null) return parent;
				factoryFn.apply(null, args);
			}
		}
	];
}