Fix systemjs-angular-loader.js for <=IE10

[nielsheeren]

"let" is ES2015 and therefore not supported by<=IE10

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[Foxandxss]

Welp, made the mistake to merge it too son. Can you please sign the CLA? Or I shall revert the commit :(

[nielsheeren]

I signed the CLA immediately after the post but didn't reply because you already committed it.