[IronPython] ThrowingErrorSink and CompilerContext

[Dan Eloff]

Internally, in compile builtin, IronPython creates a CompilerContext like:

new CompilerContext(sourceUnit, opts, ThrowingErrorSink.Default)

But ThrowingErrorSink is internal, what can I do to get a equivalent
CompilerContext?

This is blocking the _ast module from running on unpatched IronPython.

Thanks,
-Dan