Warn of potential memory leaks

[DullReferenceException]

Purpose (TL;DR) - mandatory

• Warn on the console if a potential memory leak is detected
• Allow adjusting of the leak detection threshold

Fixes #2356

How to verify - mandatory

1. Check out this branch
2. npm install
3. Call sinon.stub() 10,001 times in a loop
4. Verify that a console warning is output

Checklist for author

• npm run lint passes
• References to standard library functions are cached.

[mantoni]

Great! Thank you 👍

Can I ask you to use printWarning from our commons library?
https://github.com/sinonjs/commons/blob/e909e9746c32ba6c3c67e64b9ca5b62644cf343e/lib/deprecated.js#L48

This is safer and tests across various platforms to behave nicely.

I'm also not sure if the default threshold of 1000 isn't a bit low, since APIs like sinon.createStubInstance create lots of instances at once. What do you think @mroderick?

[DullReferenceException]

Updated, bumping default threshold to 10000 and using deprecated.printWarning instead.

[fatso83]

Great stuff and something that should have been there long ago. Not totally sold on the track() naming, though. I think it was easier to reason about what push(collection, spy) did than track(spy), so I would have preferred if the tracking bits were internal details of "pushing into internal collection", as the latter is the main point. Maybe a simple rename is all that's needed (addToCollection(spy)?). These is all quite minor details, though, and we could merge it as is.

[DullReferenceException]

Good point on naming. Renamed to addToCollection as suggested.

[fatso83]

Thanks!