car.ts

import { Inject, Injectable, InjectionToken, Optional } from '@angular/core';

import { Engine } from './engine';
import { Radio } from './radio';
import { Transmission } from './transmission';
import { Wheel } from './wheel';

const WHEELS = new InjectionToken('Wheels');

@Injectable()
class Car {
    constructor (
        public engine: Engine,
        @Optional() public radio: Radio,
        @Inject(WHEELS) public wheels: Array<Wheel>,
        public transmission: Transmission
    ) { }
}