please help while install the npm

1. A concise explanation of the problem you're experiencing.

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

npm WARN optional Skipping failed optional dependency /chokidar/fsevents:
npm WARN notsup Not compatible with your operating system or architecture: fsevents@1.2.4

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

4. The Cesium version you're using, your operating system and browser.

The fsevents module is an optional dependency for some of the build tools that Cesium uses (I think it’s Mac-only or something). Per the message, this is just a warning, and not an error. You can safely ignore it.