babel-parser(ts): Raise recoverable error for abstract interface

[sosukesuzuki]

Q	A
Fixed Issues?	Fixes #12768
Patch: Bug Fix?	Y
Tests Added + Pass?	Yes
License	MIT

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[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/39972/

[nicolo-ribaudo]

Could you rebase? CI should be fixed now.

[JLHwung]

LGTM with some edge cases and code nits.

[JLHwung]

nit: I prefer to move this branch to the else branch of isClass:

if (isClass) {
  parseClass(...)
} else if (this.isContextual("interface")) {
  throw on abstract interface;
} else {
  throw on unexpected token;
}

[JLHwung]

interface should not have a proceeding line break:

interface
Foo {}

is invalid. So it does not suffice to conclude that it is an interface declaration when we see a contextual interface name.

[sosukesuzuki]

Is there a way to avoid this lookahead?

[nicolo-ribaudo]

Maybe with a combination of lineBreak.test and this.nextTokenStart.
If it works, it would be useful to extract it to a hasFollowingLineBreak() method.

[sosukesuzuki]

It seems to work fine! da8ce69