Make compatible with Babel loose mode optimizations

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By the time https://github.com/babel/babel/pull/4850 materializes in Babel, we'll have to make some fixes here.

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Though the Babel loose mode optimizations have been merged, there hasn't yet been a new alpha version that includes it, so I tried to install from GitHub, but alas, Babel is a monorepo and NPM doesn't support it. Basically I would need to transpile some babel files myself and replace them in my local node_modules folder. I've opted to give it some more time instead and wait for the next alpha release.

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This can go ahead now since the latest alpha release: https://github.com/babel/babel/releases/tag/v7.0.0-alpha.15