[WIP] Improve error message for await-outside-async

[motiz88]

This is an attempt to implement #113. I would love some early feedback/pointers, as well as help completing the following list which reflects the current state of this branch. I only added tests for true positives and true negatives, so they should all pass despite the work being incomplete.

Examples

😎 True positives 👍

The following all raise await can only be used in async functions correctly (to my understanding).

• await promise
• return await promise
• var x = await promise
• x[await promise]
• x(await promise)
• x(await promise, 0)
• await await
• await yield
• await this
• await super
• await {}
• function* a() {yield await 0}
• await new Promise
• await.0

Non-examples

😎 True negatives 👎

The following all raise Unexpected token etc, correctly (to my understanding).

• await}
• await( - parseExprSubscripts
• await.
• await[
• await+
• `await``
• await/
• await*
• await[0;
• await(0
• await instanceof

😞 False positives 👍

The following all raise await can only be used in async functions incorrectly (only in actual non-async contexts, though).

parseExpressionStatement

• await var
• await)
• await~
• await{
• await x(
• await x((

parseSubscripts

• x[await

parseParenAndDistinguishExpression

• (await

😞 False negatives 👎

Cases which should raise await can only be used in async functions but raise some other error: None that I know of.

😎 No error at all 👌

• `await ```

[codecov-io]

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No coverage report found for master at 44d44a2.

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[hzoo]

Should we change the wording to say something like: can only be used in an immediate async function (or just to say the parent function should be async) - we could give an example too

[danez]

We moved babylon back into the monorepo at https://github.com/babel/babel/tree/master/packages/babylon.

Unfortunately pull requests cannot be migrated between repositories automatically. If this PR is still valid please reopen it there.
Thanks for your effort.