rewrite top-level this as undefined

[Rich-Harris]

We didn't catch that this wasn't already happening (pretty sure it did at one point), because Babel was doing it for us in the test (and we needed Babel, because we need to test behaviour inside arrow functions...)

[eventualbuddha]

Looks good.

[cognitom]

To bundle the code for non-Node environments, sometimes this is needed on top-level. For someone like me, this is a simple workaround for it, kind of:

doSomething(that)

Add intro option:

const code = bundle.generate({
  intro: 'const that = this'
}).code;

Then the output:

const that = this
doSomething(that)

[Rich-Harris]

For anyone else landing here -> #858

[cognitom]

Cool!