Flutter tools

Fix a typo on the README.md

We still have the --http option as a fallback for now. Once we're confident the
--no-http version works, we'll drop the --http support.

Also, create the FLX in a temp directory and then delete the temp directory
when we're done. Finally, pull the Linux artifacts from the cloud storage
bucket that the buildbot is uploading to.

add a persistent daemon/server mode to sky_tools

wrap up first daemon implementation

Start building the repository structure

...by requiring a new version of analyzer.

Also, when the subprocess is terminated unexpectedly, report the actual
problem in more detail.

abstract some OS operations

When the engine dies unexpectedly during test execution, we have to
terminate any tests running in that engine. Previously, they would just
hang. For some reason that I was never able to satisfactorily explain,
the WebSocket doesn't die in a way I can detect in this case. So
instead, we hand in a future that we only complete when we detect the
server subprocess ends.

use stack_trace to create more readable exception traces

The previous attempt to port the 'test' framework to the new framework
wasn't super-successful. This does a better job, hopefully.

skip a test on windows

This flag builds a local FLX file and pushes that to the device instead of
using an HTTP server.

increase test timeout