WebCrypto with ECMAScript 5.1?

I want to use WebCrypto, Encrypted Media Extensions, and Media Source Extensions in a context where only ECMAScript 5.1 is required.  So I need to at least use a polyfill for Promise, as it is an ECMAScript 6 feature.  Any other polyfills that I would need?  Would any of those polyfill(s) somehow introduce a security risk that would not be present if ECMAScript 6 was instead used?

Thanks,
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David Waitzman
Envieta Systems LLC