Prebuild process sometimes hangs

[asakusuma]

When adding a prebuild script that runs during prepare and uses ember-rollup/src/prebuild, calling rollup.preBuild() sometimes hangs indefinitely. The workaround is to manually call process.exit(0);. I added a console.log(process._getActiveHandles()); to debug. Here is the output. It appears that the hanging may be related to a broccoli-babel-transpiler/lib/worker.js process hanging on.

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