We originally introduced the idea of language experiments as a way to get
early feedback on not-yet-proven feature ideas, ideally as part of the
initial exploration of the solution space rather than only after a
solution has become relatively clear.

Unfortunately, our tradeoff of making them available in normal releases
behind an explicit opt-in in order to make it easier to participate in the
feedback process had the unintended side-effect of making it feel okay
to use experiments in production and endure the warnings they generate.
This in turn has made us reluctant to make use of the experiments feature
lest experiments become de-facto production features which we then feel
compelled to preserve even though we aren't yet ready to graduate them
to stable features.

In an attempt to tweak that compromise, here we make the availability of
experiments _at all_ a build-time flag which will not be set by default,
and therefore experiments will not be available ...