The "debugger" keyword is part of JavaScript, so the logical place to look is in the V8 source code.
Using the power of Code Search, we can formulate a query to pull up some places that contain the string "debugger" in them:
Depending on how much you know about compiler technology, you'll want to look for a file related to parsing. I'll leave the actual file as an exercise for the reader, as there's only a few candidate files to consider, and it'll probably help in the long run to go through the exercise of figuring it out from here.