We were writing theme-transpiler JS files to the filesystem on a per-process basis, and then immediately reading them back in. Plus, there was no cleanup mechanism, so the tmp directory would grow indefinitely.
This commit refactors things so that the `build.js` script outputs the theme-transpiler source to stdout. That way, we can read it directly into the process, and then into mini-racer, without needing to go via the filesystem. No cleanup required!
In production, the theme-transpiler is still cached in a file during `assets:precompile`
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