Jest 22 Memory Leak

Jest 22 appears to be reporting memory leaks on very minimal test suites that don't have any obvious memory leaks.

To reproduce the memory leak:

1. Install dependencies.
2. Add identifiable console.log statements to lines 54 and 66 of node_modules/jest-leak-detector/build/index.js. This will show the order of execution when jest attempts to detect leaks and garbage collect.
3. Run yarn test.

With one additional log statement at line 52 to print the value that has a weak callback attached to it, the output of the test is as follows:

➜  jest-22-mem-leak git:(master) ✗ yarn test
yarn test v1.0.2
$ jest --detectLeaks --runInBand
 FAIL  ./file-1.test.js
  ● Test suite failed to run

    EXPERIMENTAL FEATURE!
    Your test suite is leaking memory. Please ensure all references are cleaned.

    There is a number of things that can leak memory:
      - Async operations that have not finished (e.g. fs.readFile).
      - Timers not properly mocked (e.g. setInterval, setTimeout).
      - Keeping references to the global scope.

      at node_modules/jest-cli/build/test_scheduler.js:115:22

[IN JEST (line 52)] attaching GC listener to JSDOMEnvironment
[IN JEST (line 66)] checking leak
[IN JEST (line 52)] attaching GC listener to JSDOMEnvironment
 FAIL  ./file-2.test.js
  ● Test suite failed to run

    EXPERIMENTAL FEATURE!
    Your test suite is leaking memory. Please ensure all references are cleaned.

    There is a number of things that can leak memory:
      - Async operations that have not finished (e.g. fs.readFile).
      - Timers not properly mocked (e.g. setInterval, setTimeout).
      - Keeping references to the global scope.

      at node_modules/jest-cli/build/test_scheduler.js:115:22

Test Suites: 2 failed, 2 total
Tests:       0 total
Snapshots:   0 total
Time:        0.734s
Ran all test suites.
[IN JEST (line 54)] garbage collected value
[IN JEST (line 66)] checking leak
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

This shows that the weak listener is attached, the leak check occurs, then later the value is garbage collected meaning a JSDOMEnvironment is still around when leak detection occurs causing jest to report a leak has happened.

Leaks are also reported in with this code when not using --runInBand but in that case console output is not printed.