Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.
The behavior of acquire+release annotation handling has changed in
https://reviews.llvm.org/D49355 which breaks the build with the new
Clang. However, as has been pointed out, the acquire+release isn't
the right way to prevent double locking as the annotations negate
each other; the correct way is to use excludes or negative requires.
Using excludes annotations also requires using std::lock_guard instead
of std::unique_lock because the latter doesn't have the thread
annotations due to deferred locking which is not needed in Flutter and
so std::lock_guard is a sufficient alternative.
