function init(ctx, filename) {
  vm.runInContext('$traceurRuntime.setupGlobals(global);', ctx, filename);
}
A one-time initialization function for setting up the initial repl
environment. We need to do this because it runs in a separate context.

Or to be more specific, the globals from the current context are copied
into the repl context -- except for the core JS objects such as Object,
String, Array, etc.
@param {object} ctx The context to initialize.
@param {string} filename The filename to use when reporting errors.