Large memory consumption when collecting coverage for many isolates

[itsjoeconway]

We have tests in Aqueduct that spawn many isolates (>30). The current coverage strategy pauses isolates at exit and waits for tests/collection to finish before resuming each isolate so that it completes. Memory consumption gets large - I've seen it reach 10gb. We've split up tests that spawn a lot of isolates into separate files, but boxes that run automated test/coverage are still failing with out of memory exceptions.

As a trial, I forked this repository and added code to resume each isolate after it was collected. This did reduce memory consumption down to a negligible amount, but created failures later in the process. With that modification, the coverage script will simply exit after a period of time with no exception, but without the work being completed. I'm afraid I've reached the limit of my understanding here.