This seems to work fine. I think the linked report didn't work because of the incomplete selector in the previous line:

fmt.
var x = []string{fmt.<>}

In this case, the second line is completely missing from the AST, so it is trickier to fix.

Is there a reasonable expectation that completion work if the code above the point at which completion is expected isn't syntactically correct (as is the case in the linked vscode-go issue).

Yes, I think we want to do the most we can to get completion to work, even on broken code. I've added a gopls-parse label that we can use to mark such issues.

Change https://golang.org/cl/216484 mentions this issue: internal/lsp/cache: improve completion after dangling selector