Firefox does not preserve datachannel.send() call order when Blobs are sent (relative to other data sent).

STRs: https://jsfiddle.net/jib1/v2ctmg5L/ and web-platform-tests/wpt#18770.

For details see w3c/webrtc-pc#2215.

The order is preserved when both data1 and data2 are Blobs https://plnkr.co/edit/7oYM0i?p=preview

Marking as P2. :jib, please change if you disagree.

This seems to be because we're doing a couple of extra dispatches when sending a Blob. These extra dispatches seem to be related to a performance optimization that offloads the reading of a Blob to another thread:

https://searchfox.org/mozilla-central/rev/f1e99da78fe6c3c68696358dac06aed90f8112d3/netwerk/sctp/datachannel/DataChannel.cpp#2765

We definitely should be waiting until after the blob is sent before sending other data.

There's a test-case in webrtc/RTCDataChannel-send-blob-order.html that fails because of this bug, but now this test-case intermittently "succeeds" on fission for some reason.

FWIW https://jsfiddle.net/jib1/e0z61uo2/ shows WebSocket does not have this problem.

But until Chromium adds blob support (webrtc 2276) this is probably low priority.

Bumping back to P2 due to recent activity in other browsers.