Address comments from https://dart-review.googlesource.com/c/sdk/+/174471

* Rename `completer` to `result` (its role, rather than repeating its type).
* Change × in non-dartdoc comment to the actual character.
* Throw on an unexpected source address argument type.

Also handle failing unix socket connections better.
The current code did not account for all possible return values
from the native connect functions. Likely, those other values never
occurred, but unless it's proven that they can't, not even in unsound
mode, the code should be prepared for them.
(And added missing `return` to the native code).

TEST= Refactoring, no change to tests.

Change-Id: Ie27670f62ae6ecc64dc045c28869e3d5ab218fda
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175042
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>