Follow-up work for compiling node_modules

[gaearon]

Follow-up for #3776.

First thought: we should have an end-to-end test that depends on ES6 package (e.g. chalk@2.x). 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?

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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 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

[EnoahNetzach]

A couple of considerations/questions:

1. I propose we create a "fake" ES6 package crafted so that every main ES6+ syntax feature is certainly included.

2. The "too old" heuristic is to be done on the node_modules if I understood that right?

3. In order to verify it doesn't break third-parties, we could just add an integration test that relies on the "fake" ES6 package, similarly to what we already do.

4. In order to verify that helpers are being shared, could a regex be sufficient? What if another dep ships with a copy of that helper?

[gaearon]

I propose we create a "fake" ES6 package crafted so that every main ES6+ syntax feature is certainly included.

Yes please.

The "too old" heuristic is to be done on the node_modules if I understood that right?

Yes.

In order to verify it doesn't break third-parties, we could just add an integration test that relies on the "fake" ES6 package, similarly to what we already do.

Yep.

In order to verify that helpers are being shared, could a regex be sufficient? What if another dep ships with a copy of that helper?

I think it's okay to just verify this manually. I don't expect this to break once we make sure it works.

[gaearon]

I don't think these are relevant?