The optimism package no longer knows anything about Fibers, but it does
export various helpers for managing execution contexts, one of which
(noContext) allows us to censor the current context for the duration of a
function call. By wrapping Fiber.yield with noContext, we keep distinct
Fibers from accidentally registering cache dependencies on one another.
Related to https://github.com/meteor/meteor/pull/8728#issue-232369984,
though these changes apply only to the meteor/tools codebase. We still
need to make similar changes to the babel-compiler package so that
application code will benefit.
The assumption that we're using Node 8 also allows some simplifications to
the runtime polyfills that we use.
