coverage issues on canary #97

• Version: v12.0.0-pre
• Platform: OSX
• Subsystem: process

I'm trying to test some changes @schuay, @hashseed, and I have been working on for block coverage, and I noticed some regressions on this branch with relation to coverage handling:

1. in internal/process/coverage.js, coverageDirectory is now undefined when entering the writeCoverage method; this suggests that writeCoverage is perhaps happening in a different threads than before?

2. if I go ahead and initialize coverageDirectory, coverageConnection.dispatch appears to hang indefinitely.

Not sure if this is due to upstream changes in v8, or downstream changes in Node.js.