Configuring `lib.externals` for third-party library

I always get error on compiling:

BUILD ERROR
'letProto' is not exported by node_modules\rxjs\operator\let.js
Error: 'letProto' is not exported by node_modules\rxjs\operator\let.js

Here is my source code: https://github.com/piernik/alert-error

[isaacplmann]

I get the same error. I can fix it by adding this to ng-package.json:

{
  "lib": {
    "externals": {
      "rxjs/operator/let": "Rx.Observable.prototype"
    }
  }
}

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