fix(router): prevent memory leaks by removing app references from $router.apps once app destroyed (Fixes #2639)

src/index.js

@@ -91,6 +91,11 @@ export default class VueRouter {
 
     // main app already initialized.
     if (this.app) {
+      app.$once('hook:destroyed', () => {
+        // Clean out app from this.apps array once destroyed
+        const index = this.apps.indexOf(app)
+        if (index > -1) this.apps.splice(index, 1)
+      })
       return
     }