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.

This adds a new navigationMode to the MediaQueryData that indicates how focusable controls should behave under different navigation modes, currently with two modes: NavigationMode.traditional, and NavigationMode.directional.

It may seem like focusing a disabled control is not desirable, but this is useful for user interfaces that use DPAD navigation because if a control gets disabled, losing focus is disruptive to the user, and it is difficult to control where the focus will end up unless it is done explicitly.

This reverts commit f77c669d.

In #55864 a race condition was described where a done event is received before we finish connecting. This cannot happen, since async functions begin synchronously and the flag isWaitingForVm is tripped immediately, keeping onDone from exiting.

The CMake plugin build wasn't setting visibility to hidden by default,
which meant that plugins exported everything by default. This would make
bad interactions between plugins much more likely; only the intended API
should be exported by the shared library.

When you run this, bash complains that `local` is only valid in functions. Gets rid of that extraneous warning.

Just adding some more details to the snippets README.md file about testing snippets.