Error in iOS Safari 12

[hgcummings]

On iOS Safari 12, audio processors can't extend AudioWorkletProcessor because it's not a class with a prototype object.

Error and stack trace

TypeError: The value of the superclass's prototype property is not an object.

1. Eval Code
2. eval
3. about:blank:2:2231
4. (anonymous function) — index.js:86
5. promiseReactionJob

This is the offending line https://github.com/GoogleChromeLabs/audioworklet-polyfill/blob/master/src/index.js#L86. which is (unsurprisingly) the line that evals the processor.