Erros após utilizar

Reparei que no vídeo não baixamos a biblioteca "vue-router" via command line npm, porem mesmo assim no curso funcionou, será que foi importado no curso e por isso não está funcionando aqui?

main.js :

import Vue from 'vue'
import App from './App.vue'
import VueResource from 'vue-resource';
import VueRouter from 'vue-router'
import { routes } from './routes';

Vue.use(VueResource);
Vue.use(VueRouter);

const router = new VueRouter({routes});

new Vue({
  el: '#app',
  router,
  render: h => h(App)
})

router.js :

import Cadastro from './components/cadastro/cadastro.vue';
import Home from './components/home/home.vue';

export const routes = [
    { path: '', components: Home},
    { path: '/cadastro', components: Cadastro}
]

Erros:

Uncaught Error: Cannot find module "vue-router"
    at eval (main.js?3479:1)
    at Object.<anonymous> (build.js:1012)
    at __webpack_require__ (build.js:679)
    at fn (build.js:89)
    at Object.<anonymous> (build.js:860)
    at __webpack_require__ (build.js:679)
    at build.js:725
    at build.js:728
--------------------------------------
[WDS] Errors while compiling. Reload prevented.
errors @ client?cd17:161
onmessage @ socket.js?e5d0:41
EventTarget.dispatchEvent @ sockjs.js?3600:170
(anonymous) @ sockjs.js?3600:887
SockJS._transportMessage @ sockjs.js?3600:885
EventEmitter.emit @ sockjs.js?3600:86
WebSocketTransport.ws.onmessage @ sockjs.js?3600:2961
---------------------------------------
./src/main.js
Module not found: Error: Can't resolve 'vue-router' in '/Users/coeficiente/Documents/vue-alura/src'
 @ ./src/main.js 4:0-35
 @ multi (webpack)-dev-server/client?http://localhost:8080 webpack/hot/dev-server ./src/main.js

Erro que aparece na tela do navegador:

Failed to compile.

./src/main.js
Module not found: Error: Can't resolve 'vue-router' in '/Users/coeficiente/Documents/vue-alura/src'
 @ ./src/main.js 4:0-35
 @ multi (webpack)-dev-server/client?http://localhost:8080 webpack/hot/dev-server ./src/main.js

Obrigado pela atenção!