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We use babel to test itself. Our fixture tests are hand written "source files" (called actual.js) that are then run through babel and byte checked against our expected output files (called expected.js). For advanced runtime tests, we run exec.js code, which is source code that is run through babel then executed as actual JS code with tests assertions.
We currently use Mocha unit tests with a large dataset of expected inputs and outputs for transformations.
For more info on what Justin said: github.com/babel/babel/blob/master/CONTRIBUTING.md
These are all fixture tests that some people know now as "snapshot" tests (we have our own naming, etc)
We're working with folks on TC39 like Leo, Rick, and Mike at Bocoup to get test262 tests in Babel - https://github.com/babel/babel/issues/4987. And ideally we would run Babel on itself before publishing https://github.com/babel/babel/issues/6134 as well as have smoke test for top npm packages. (these are all important contributions someone could make)