This adds logic to download and use the sky_snapshot binary from
Google cloud storage when running the 'sky_tools build' command.
The downloaded binary is put into lib/cache/... The binary is
chosen to match the REVISION in the sky_engine package in the
packages directory of whichever package the user wishes to
build a flx from.

Known issues:
*) Assumes linux-x64 host
*) Assumes download will always produce valid executable
*) No clearing of stale cache entries

With this patch, we can switch over to using this command from the build system
in the sky_engine repo.

This command will produce an flx package. Currently, this command doesn't work
because we don't have the Flutter compiler downloaded from Google storage yet.
A future patch will make that happen.

This stray line was causing an analyzer error.

Add very-verbose flag and automatically log some more process-related things in verbose and very-verbose modes.

Also applies autoreformatting to init.dart.

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

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