`.callServiceExtension` test failing for WebDev

[elliette]

DWDS changed how it connects to the VM Service - via DDS instead of directly: #2388

As part of that change, the .callServiceExtension test case began failing for WebDev.

From what I see on #191, I'm not sure this is even necessary, since it was added to support Flutter (but Webdev is no longer(?) used to run Flutter apps).

I've tested Webdev locally and everything seems to be working, so I don't think it's a high priority to fix the test.

[elliette]

Figured out what the issue was, should be fixed in #2388