1. Updates based on discussions with Dart team
See https://github.com/dart-lang/www.dartlang.org/issues/1496
* Removed *.precompiled.js - not generated by dart2js any more
* *.js_ → *.part.js
Also:
2. Added guidance comments
3. doc/ → doc/api as manually written documents may be in doc/
4. Added reference to appropriate Dart web site page
Modify patterns to explicitly match directories
Modified patterns to match files created by tool in Dart SDK 1.12:
- Added patterns matching files listed in https://www.dartlang.org/tools/private-files.html
- Added *.js, removed *.js_, *.dart.js to match files created by dart2js
- 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.
