Fixes #812.

Previously, this code

    module.exports = moo();
    function moo() { return __DEV__; }

would be transformed to

    module.exports = moo();
    function moo() { return "production" !== process.env.NODE_ENV; }

Now, it's transformed to:

    if ("production" !== process.env.NODE_ENV) {
      var moo = function() { return true; };
      module.exports = moo();
    } else {
      var moo = function() { return false; };
      module.exports = moo();
    }

which reduces the getter cost to one test at require time instead of inline for every `__DEV__` check, warning, and invariant.

The unminified build is about twice as large now (but it's about the same after gzipping) and the minified build is the same size.