Fix outdated mini_file_server.dart

[stevenanthonyrevo]

Issue - dart-lang/site-www#2646 - Propose to fix mini server example continuous loop on the connection.

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[stevenanthonyrevo]

@googlebot I signed it!

[stevenanthonyrevo]

Propose to fix mini server example continuous loop on the connection.

[stevenanthonyrevo]

To fix blocking variables with a standard "string" message

[stevenanthonyrevo]

@kwalrath If you would be kind to see my revisions regarding the type difference.

[kwalrath]

I think what we need to do is just copy https://github.com/dart-lang/site-www/blob/master/examples/httpserver/bin/mini_file_server.dart to this repo. That'll get rid of some problems like the unnecessary import of dart:async.

(It's kind of terrible that we have the code in 2+ places. I believe that @RedBrogdon will make sure this duplication gets fixed, one way or another.)