I got the package file
You are referencing the source of the
JSONLoader. This is written in ES6 using a modules pattern (which is what the
import token is for), that is bundled into the
build/three.js that you are referencing already. The problem arises as it's not possible to use modules in the browser (..yet).
Simply put, you do not need to reference
/src/loaders/JSONLoader.js, the JSONLoader is included by default.