Aliasing with `as` attribute doesn't work in Firefox and Safari

[antonmoiseev]

document.currentScript doesn't contain as attribute while inferring module ID:

function _inferModuleId() {
  var script = document.currentScript;
  if (script.hasAttribute('as')) { // Always `false`in Firefox and Safari
    return script.getAttribute('as');
  }
  //...

[antonmoiseev]

Here is a small demo app. The problem is reproducible only within HTML Imports. According to the polyfill docs document._currentScript should be used within the browsers without native HTML Imports support. Looks like it fixes the problem, please see README.md of the demo app.