This command uses package:test to run Dart tests with sky_shell.  For this to
work, we need https://github.com/dart-lang/test/tree/hacky-loader-hook to land.
We're also not smart enough to find sky_shell ourselves yet. Instead, we take
the path as input using an environment variable. Eventually, we'll be able to
get the sky_shell executable from package:sky_engine, but we don't yet ship
that executable.

This is needed to make pub run sky_tools:sky_server work.
Partial fix for https://github.com/domokit/sky_engine/issues/539

TBR=abarth

Add a travis badge.

This feature appears to be causing problems with OkHTTP's cache.

This patch switches sky_server.dart to use shelf rather than http_server.