Invalid system.js build

[joewoodhouse]

Stencil version:

 @stencil/core@1.7.5

I'm submitting a:
[x] bug report
[ ] feature request
[ ] support request => Please do not submit support requests here, use one of these channels: https://stencil-worldwide.herokuapp.com/ or https://forum.ionicframework.com/

Current behavior:
When I run a build, one of the built system.js files seems to be invalid. Specifically the minified JS generated contains a call to a function __extends_1 that does not exist. Trying to load this build in the browser causes errors.

Expected behavior:
System js built file should not contain errors.

Steps to reproduce:
Run a build

Related code:

p-e1fea07b.system.txt

search for __extends_1

Other information:
I assume this is some sort of rollup issue, but I've no experience with rollup so difficult for me to investigate myself.

[joewoodhouse]

Think this is probably the same issue as this?

#2013
microsoft/TypeScript#35108