Because the current environment (browser or Node) does not support ES6 code, so some shim and polyfill (IE need) to ES6 write code can be converted to ES5 form and can run properly in the browser.
From the above figure we can see in the Es5 browser need the following module loader:
Systemjs- generic module loader, support for AMD, CommonJS, ES6 and other formats JS module load.
Es6-module-loader- ES6 module loader, systemjs will automatically load this module.
Traceur- ES6 transcoder, convert the ES6 code to the ES5 code supported by the current browser, and systemjs will automatically load the module.