Add jest:detectleaks npm script for Jest memory leak testing.

[ntwb]

Follow up to #3069/ #3121

[ntwb]

I've created #3124 to list, and seek help in fixing the current 17 tests that fail

[hudochenkov]

What weak for? Is it peer or optional dependency of Jest?

[ntwb]

My bad, I should have added a note about weak, Jest Leak Detector requires weak:
https://github.com/facebook/jest/blob/master/packages/jest-leak-detector/package.json#L14

[hudochenkov]

Found it https://github.com/facebook/jest/blob/19376c1e1e8a77717fbb7a216df92087eddbd277/packages/jest-leak-detector/src/index.js#L39-L51 Quite strange approach for dependencies.

[hudochenkov]

LGTM.

[ntwb]

I think it's due to the mono repo Facebook use:
• ~35 npm modules in a single repo https://github.com/facebook/jest/tree/master/packages
• Lerna is what's used to manage this repo setup https://lernajs.io/ - "A tool for managing JavaScript projects with multiple packages."

[hudochenkov]

It shouldn't be because of Lerna. I use it at work and any package could have whatever dependencies it wants.

[SimenB]

It's because of jestjs/jest#5169, fwiw