watchman can't be installed via npm, so it can't easily be integrated as a widely-used dependency.

Our statistics on ember-cli posix consumers with watchman installed is sufficiently high to assume this is not a barrier for our users.

Chokidar pros:

bundles with npm, happy path has a low barrier of entry
good performance on supported platforms

Chokidar cons:

subjects users to longer installs
brittle native dependency fun
unhappy path is painful

the last too are currently show stoppers IMHO

Watchman pros:

extremely fast (especially at filtering out unwanted things like tmp / node_modules)
system wide knowledge which allows rolling up and merging of event streams (it is not uncommon for people to be running multiple ember-cli doing different things on the same project test / serve / build, it is very nice that watchman aggregates and combines these two)
known to mitigate several scenarios that cause FSEvents to get into a bad state (maybe Chokidar also does this)
is able to handle watching the root of a cli project, using its native filtering to drop the chatter ./tmp changes.

Watchman cons:

requires a one-time external dependency install
users who don't have it installed, fall back to raw fs.watch (which a helpful message that suggests they upgrade)
doesn't yet work on windows (rumor has it this will change Alpha Windows support facebook/watchman#19)

Future work planned that depends on watchman:

While working with pure FS watching, we have observed that a system. especially a system under load will drop intermediate FSEvents, which means treating the change stream as a source of truth for changes is flawed.

Currently broccoli mitigates this issue, by following up with a series of stats to build an accurate source of truth. Luckily on most system stating is insanely fast, tens of thousand of files can be stat'ed near instantly.

As watchman maintains internal change state (by both watching the stream, and if dropped changes are detected crawling/stating), we plan to utilize its state via is query API, to avoid current duplicating work in broccoli.

Replace sane or polling and favor chokidar #4503

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