Define an operation to make a promise as handled

[yutakahirano]

Split from whatwg/fetch#959.

Streams and Fetch sometimes manipulate promise.[[PromiseIsHandled]] to suppress unhandled rejection error reports. We'd appreciate if this is defined here so that they don't need to manipulate the internal property directly.

@annevk, @domenic, @ricea

[Ms2ger]

Are all cases handling actual Promise objects, not IDL Promise<T>-typed values? Maybe it would make sense to add an abstract operation to ES directly.

[domenic]

They are actual Promise objects, but only because of the bad layering in Streams. After whatwg/streams#963 they should become Promise<T> objects. (And in the meantime, treating them as if they were such seems OK-ish.)