demo/repl.js: update to use '$traceurRuntime'.

This is a follow up to af15b37c95b014a473176df2dcf4687b83f2d825

Closes #273
Make traceur modules available during traceur load.

The use case is to be able to "import * from '@iter';" during module
load. In addition, this will allow all compiled files to use the same
construct, provided that 'src/runtime/runtime-modules.js' is linked in
as well.

Closes #274