Module names are now normalized as URLs in the default System implementation, with locate simply the identity operation.
.js extensions are also no longer added by default.
These changes are part of the transition into the new specification work. See the discussion at whatwg/loader#52 for further information.
.js extension adding can easily be added back if needed with a custom hook.
__moduleAddress is no longer provided, just __moduleName now that this is a URL
Loader builds no longer include the promises polyfill, but do include the URL polyfill.
baseURL is no longer implemented. It can be replaced by wildcard paths - System.paths['*'] = '/path/*'