VirtualDirectory.serveFile should return a Future

[DartBot]

Originally opened as dart-lang/sdk#16924

This issue was originally filed by [email protected]

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VirtualDirectory.serveFile of http_server library should return a Future that completes when the file has been successfully served.

It is a small change, so I made a CL: https://codereview.chromium.org/171363006

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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cc @skabet. Removed Type-Defect label. Added Type-Enhancement, Pkg-HttpServer, Area-Pkg, Triaged labels.