Update idlharness to support constructor operations.

[Ms2ger]

See whatwg/webidl#778.

[foolip]

Ouch, this regressed all tests depending on html.idl. You can see it in https://github.com/web-platform-tests/wpt/runs/207949624 right on this PR but I didn't check that when reviewing. You can also see all the broken tests live on wpt.fyi.

@web-platform-tests/wpt-core-team can someone quickly revert this? I don't have access to a computer with a command line this weekend, but can follow up with "stop this from happening again" types of bugs next week.

@lukebjerring a case for regression detection here :)

[foolip]

What the failure saya::

FAIL message: promise_test: Unhandled rejection with value: object "WebIDLParseError: Syntax error at line 1471, since `interface CustomElementRegistry`:
 define(DOMString name, CustomElementConstructor constructor, optional ElementDefinitionOptions
                        ^ Trailing comma in arguments list"

This looks like a webidl2.js regression to me.

[foolip]

For anyone who also didn't know, @jgraham pointed out that it is possible to revert from the GitHub UI. It shows up next to the "Ms2ger merged commit 41a1425 into master 21 hours ago" event.

[foolip]

I've filed w3c/webidl2.js#402.