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.
fixes#57802
For users without git installed, flutter on windows bundles mingit. This needs to be added to the PATH in the main script, not the executed script, so that it is available to the tool
