Consider Blob.fromStream (returns a Promise<Blob>)

I'm filing this as a tracking issue, although I don't think it's particularly urgent. But maybe web developers have run into this more, in which case hearing from them would be great.

Anyway, right now, to convert a ReadableStream to a Blob, you have to do

const blob = await new Response(stream).blob();

which is pretty silly. Just as blob.stream() was added to avoid the silly new Request(blob).body pattern, perhaps we should consider adding a promise-returning Blob.fromStream() so you could do

const blob = await Blob.fromStream(stream);

There is a bigger savings if we add an options parameter:

const blob1 = new Blob([await new Response(stream).blob()], options);
const blob2 = Blob.fromStream(stream, options);

Points to consider:

• We could instead make this stream.toBlob(), but I feel the layering of keeping streams ignorant of blobs is a bit cleaner.
• We may not want to introduce this because it's better if people avoid using blobs? (Or is it just blob URLs that we dislike?)
• If someone wanted a File, we could either add File.fromStream(stream, fileName, options) or we could have people do new File([Blob.fromStream(stream)], fileName, options). Probably File.fromStream() is nicer.
• This does not allow creating a blob that represents an "in progress" stream, which I've heard people have wanted in order to have self-referential blobs. That would require new concepts and infrastructure. This proposal is basically just exposing existing infrastructure in a more straightforward way.

The issue names no repository files or tests. Start by comparing the current Blob, ReadableStream, and Response(...).blob() APIs described here; done would require a settled API design for Blob.fromStream, its options, and the related File question, followed by the applicable specification and conformance tests.