Update Dart.gitignore

- add the `build/` directory to the .gitignore list (now created by pub).
- add the `.dart.precompiled.js` files (created by dart2js) to the ignore list.

And, sort and re-order the file.
Add a support for packer

http://www.packer.io/
Create OpenCart.gitignore

Ignore OpenCart config files, images, cache and logs.
Merge pull request #993 from tacahilo/add-packer

Add a support for packer
Merge pull request #991 from devoncarew/patch-1

Update Dart template for pub, dart2js
Merge pull request #1001 from teslitsky/opencart

Add OpenCart template