The CFE now logs to stdout by default when compiling a program which has
non-null-safe dependencies. Since flutter_tools has not yet migrated, we
need to suppress this message when compiling the tool.
Fixes https://github.com/flutter/flutter/issues/74366
Currently an invocation of flutter/dart will always attempt to acquire a lock. This can pose problems for tools that attempt to run multiple dart/flutter instances.
Instead update the lock logic (on Linux/macOS) so that we only attempt to acquire it if an update/snapshot needs to be performed. To avoid repeatedly performing downloads/snapshots if multiple flutter/dart invocations are fired off concurrently when an update needs to be performed, do a second check of the download/snapshot condition after the lock is released.
Additionally, moves all of the building/debug output to stderr on both the bash and batch scripts. This allows machine mode consumption of the tool to ignore needing to parse/handle the rebuild messages.
This makes the flutter and dart scripts invoke their batch file equivalents if running under MINGW (i.e. git-bash) on Windows.
This allows for proper locking, and makes sure that people aren't using two different (and non-mutally-aware) locking systems when running flutter on Windows.
I also fixed a couple of places where we look for MINGW32, which fails under MINGW64. It just looks for MINGW now.
This reverts the flutter command to use shlock when flock isn't available. It seems that the mkdir method isn't as reliable as we want. I think that this is because the trap isn't always be executed, which is why I think that shlock uses PIDs to help it be more reliable. Unfortunately, that means that we're back to not working over network shares (which is where things were before I moved to the mkdir method, so not really a regression). I did leave in the mkdir method for platforms that have neither flock nor shlock (which should be very few and far between, but still), so at least we'll do some locking there now.
