Update the flutter script's locking mechanism and follow_links (#57590)
Update the flutter and dart scripts' locking mechanism and follow_links function to be more robust and support more platforms.
This adds support for using mkdir as a fallback if the system doesn't have flock instead of using shlock, since shlock doesn't work on shared filesystems.
It also fixes a problem in the follow_links function where it failed when the link resolved to the root directory.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/379e11b64194bd9b1b4a159134476d69856f8d11
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