Upgrade to Babel 7

[stasm]

Babel 7 is still in beta but very much usable. Let's use this issue to track what changes are needed on our side when we upgrade.

Improvements:

• Remove the transform-builtin-extend plugin. (Add support for extending builtins babel/babel#7020)
• OK to use destructuring in for await loops. (Fix async-to-generator ForAwait transform babel/babel#5932)
• Maybe drop stage-3 in favor of @babel/plugin-proposal-async-generator-functions?

Open questions:

• Will the logic of reading .babelrc files change? (How do we want to handle .babelrc lookup in 7.x babel/babel#6766)