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.
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