The test suite uses our Promise polyfill

[pygy]

The logic in ./promise/promise.js picks up the global window defined by the ./tests/test-api.js, doesn't find a Promise implementation, and goes on to use our polyfill instead of native promises.

Adding native promises to ./test-utils/broswerMock.js would solve this regardless of any future change in test loading order.

[pygy]

I fixed this in the v2.x branch, along with a perf regression in the test suite.

@StephanHoyer @dead-claudia Do you see this going into next as well?

ed2365d