Follow-up for #3776.

First thought: we should have an end-to-end test that depends on ES6 package (e.g. [email protected]). After the build, we can run ESLint in ES5 mode with no rules on the compiled bundle so that end-to-end test fails if we accidentally include ES6 code.

There may be more things we need to test. I’m not sure. Thoughts?

My original todos:

• Consider opt-out heuristics (e.g. "too old" engines field that implies we don't need compilation)
• Verify this doesn't break third-party code
  • Decide on the behavior for sourceType [8.x] Gain consensus on path forward for "sourceType" for parsing modules vs scripts babel/babel#6242
• Verify generators and async/await work in deps
• Verify helpers for the above are being shared (e.g. runtime transform)
• Verify .babelrc in packages gets ignored
• Create end-to-end tests for these situations