add import.meta

[mysticatea]

This PR adds import.meta support into acorn. It represents import.meta as a MetaProperty node.

Upstreams:

• https://github.com/tc39/proposal-import-meta
• Add initial tests for import.meta tc39/test262#1888
• Normative: add import.meta tc39/ecma262#1892
• Add import.meta to ES2020 estree/estree#210

[marijnh]

Thanks! I'm waiting for the ESTree thing to merge again.

(Yeah, I know, #890 and #890 are also still stuck, after way too long, but supposedly there's interest from @nzakas and @RReverser to try and find a way to move ESTree decisions forward more effectively.)

[RReverser]

We have already agreed on using MetaProperty for meta properties, so there is nothing really new on ESTree side here and this should be good to go. (the only upstream question is whether we want to list every meta property explicitly in the spec)

[RReverser]

Do we have same error in new.target? We probably would want these error messages to align.

[mysticatea]

I have pushed 0164a93 to align the error messages.

For a reference, "'import.meta' must not contain escaped characters" is the same message as Chrome 80.

[RReverser]

I don't think it matters much to align with some engine, but internal consistency is important. Thanks for fixing!

[wizzfile]

mysticatea:import-meta