Add in rule for .buildlog

- 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.

.gitignore uses `#` for comments, not `//`

Removed space from end of line which prevented the `pubspec.lock` from being ignored.