Specifically Squeak... I wonder if it is built with Squeak?
No, Eclipse.
It is an Eclipse plugin, yes, but it could still be written in Squeak?
Eclipse was originally built in smalltalk…
The original version of eclipse was written in smalltalk…
Hmmm. I assume you're referring to VisualAge. It's a bit of a stretch to call that "the original version of Eclipse". A predecessor, yes - but the only thing the two have in common is that they were both IDEs.