Building crashes

[ccantill]

We're getting errors like the following after the latest update:

Error on fetch for npm:Base64@1.0.0.js at file:///D:/jenkins/workspace/WebPortal-Full/jspm_packages/npm/Base64@1.0.0.js
    TypeError: Cannot read property 'file:///D:/jenkins/workspace/WebPortal-Full/jspm_packages/npm/Base64@1.0.0.js' of undefined
    at D:\jenkins\workspace\WebPortal-Full\node_modules\systemjs\dist\system.src.js:3377:23
    at D:\jenkins\workspace\WebPortal-Full\node_modules\systemjs\dist\system.src.js:3696:18

In 1a6721d there's a cachedFetch that isn't bound to the loader. The "this" in that function will therefore revert to the global object, so then when "loaderFetch.call(this, load)" is invoked in cachedFetch the "this" on that call is the wrong one.