Support parsing ExportDefaultSpecifier and ExportNamespaceSpecifier

[benjamn]

This pull requests adds support for parsing two new export ... from syntaxes:

// New ExportNamespaceSpecifier syntax:
export * as ns from "module"

// Equivalent to:
import * as _ns$0 from "module"
export { _ns$0 as ns }

as proposed here, and

// New ExportDefaultSpecifier syntax:
export def from "module"

// Equivalent to:
import _def$0 from "module"
export { _def$0 as def }

// Also equivalent to:
export { default as def } from "module"

as proposed here.

Note that the _foo$0 identifiers above are meant as unique temporary variables that are no longer necessary with the new syntax.

Babylon also parses these syntaxes, so you may want to examine Babylon's output in ASTExplorer to compare it with the tests included in this pull request.

The proposals are not especially controversial, and are unlikely to change as far as the parser is concerned. However, the proposals are not yet final, so it seemed like a good idea to keep them behind the options.exportExtensions flag, even though these changes should be backwards-compatible with all existing export syntax.

cc @jdalton @RReverser

[benjamn]

Here's the Babylon version of this functionality, for reference.

[marijnh]

Hi. Please implement and distribute this as a plugin until the proposal gets to a later stage. To avoid the headache of tracking changing proposals, we're not putting anything in the main distribution until it reaches stage 4.

[benjamn]

Ok, that's fair. I'll keep rebasing this branch periodically until these proposals reach stage 4.

[jdalton]

👌 They're currently stage 1. I have high hopes they'll advance since they aren't super controversial.

[benjamn]

Perhaps @leebyron would like to offload some of his championship responsibilities? 😉

[jdalton]

Now that export-ns-from has TC39 consensus we should revisit this in acorn.

Update:

Opened #742.

[marijnh]

I still see 'stage 1' on the proposal?

[jdalton]

@marijnh That is incorrect. Reviewing the notes from the Sept TC39 meeting consensus was established on a Stage 3 manner, which will then be merged once implementations are there.

[marijnh]

All right, but stage 3 is still not stage 4

[jdalton]

@marijnh Could we please move comments to #742.