Use API functions based on availability

Hi,
I'm using es6-module-loader in a custom javascript environment and made these changes to avoid errors.
I have a window object, but do not have a document.getElementsByTagName, I have an XMLHttpRequest but no require.

What is outputting a void in the System.register name? That should never happen and if it does that is the wrong output mode in Traceur (this sounds like the Traceur `register` output).