Occasional completion (silent) failures

E.g.

```
LimitedWr   // should complete to LimitedWriteStream but doesn't

// but it's there:
LimitedWriteStream.filenameSymbol
> /home/dlm/share/superc/SCClassLibrary/Common/Streams/IOStream.sc
```

It's not the only time I've seen an expected completion fail to materialize. So I'm opening this issue just to have a place to catalog them, and hope that a pattern emerges.