Update Dart.gitignore
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.
chhsieh/gitTest_1,bruennijs/ise.cppworkshop,szymanskirafal/gitignore,szymanskirafal/gitignore,chhsieh/helloworld,bruennijs/ise.cppworkshop,bruennijs/ise.cppworkshop,szymanskirafal/gitignore