Standard-minifier can't minify the npm websocket package (with ES6 syntax)

In my meteor project, I use the npm package, which depends on the engine-io package, which depends on the ws package (v2.6.1). Inside of the ws package, the files are written in ES6, for example:

class WebSocket extends EventEmitter {
  // ...

When I run meteor build . or deploy the app via scalingo, I got the error saying that UglifyJS can’t recognize some tokens (ES6 syntax).
If I switch to use the abernix:standard-minifier-js package instead of the default standard-minifier, I can successfully build and deploy the project. But when I try to open the app in IE11, I got another error yielding Invalid character on the ES6 syntax. So my guess is the abernix:standard-minifier-js package is not stable enough to be used in various browsers.

So how can I keep the standard-minifier package while still be able to build the project ? Or how can I use abernix:standard-minifier-js but make it work on IE11 ? Any suggestions / comments are welcome !

Running 1.5 I haven’t seen the initial error, but I do see the IE11 error here as well. Anyone else come ups with a fix for this?