change setup script; fox bugs with dart analysis

Some changes to be able to develop spark in spark. Specifically, we convert the symlinks in the packages directory into real files as part of the `./grind setup` step. It lets the analyzer see these packages. I'm not 100% what other side effects this might have.

- fix an issue where derived files (files in the `build` directory) where being analyzed by the dart builder
- remove `.js.map` files after a deploy. For some reason the deploy process is creating dozens of large map files, which slows down the editor's launch (we parse these files).
- copy the `app/packages` directory over the `packages` directory. This reifies the symlinks. This is so that spark - as a chrome app - can see the file contents in the `packages` directory. Currently chrome apps don't traverse symlinks, and the packages directory content is not visible to the dart analyzer.

@dinhviethoa