If we ever load javascript (uniload load or package.js parse) outside of
a buildmessage job and there's a syntax error, FancySyntaxError gets
thrown instead of properly processed. Now, we shouldn't do that (we
should only load JS inside a buildmessage job!) but our codebase isn't
currently up to that standard, so at least ensure that there's some
level of useful syntax error (albeit with an ugly internal stack trace
attached) in this case.
Port a simplified version of Meteor.EnvironmentVariable and
Meteor.bindEnvironment to fiber-helpers.js to deal with this.
Identify uses of fiberHelpers.inFiber and switch them to either
fiberHelpers.bindEnvironment (if the callback they are wrapping is
semantically "part of" the context that creates the callback) or
fiberHelpers.inBareFiber (otherwise).
Without this, concurrency was causing the wrong buildmessage message
sets and jobs to be active when builds yielded.
