Not sure this is a devtools or v8 bug, but I can repro this consistently on canary.
Host: OSX, version: 63.0.3235.0 (Official Build) canary (64-bit)
Can repro both on M60 and M63.

Repro steps:
1) open https://perfetto-ci.appspot.com/
2) Open devtools, open sources, and load /static/script.js
3) Add a breakpoint inside GetTravisStatusForJob(), say on line 45.
4) Reload the page, the breakpoint will be now hit.
5) Insert the following code at some point in that function (again, line 45 works):
   console.assert(jobId != "1234");
6) Save and press continue.

At this point the renderer will crash in
v8::internal::(anonymous namespace)::Invoke

Crash ids:
  crash/620e17ebc074037d
  crash/66e39dc0eff13dfe