Typescript error "x" is not a module

Question

I'm building an app with PixiJs and Angular2 and I'm trying to import the pixi typescript definitions (https://github.com/pixijs/pixi-typescript).

I want to extend the Pixi Sprite type, but I am unable to import the library.

Here is the code:

import { Sprite } from '../../assets/pixi.js';

export class test extends Sprite {
     //...
}

This results in the error "File 'C:.../src/assets/pixi.js.d.ts' is not a module. at line 1 col 24"

The path is definitiely correct. What am I doing wrong and how can I fix this issue?