Followup to https://github.com/dart-lang/site-www/issues/444#issuecomment-346560788

Our typical workflow has JS and related build outputs in the `build` directory.

Most users are on new SDKs, so we don't have to worry about these ignores.
Most users don't generate docs locally. If they do, it's easy to add the entry to their .gitignore later.

Dart SDK 1.20 no longer creates `packages` directories. Added comments to `pub` files section to differentiate  what is needed for >= SDK 1.20 and what is additionally needed for older SDK versions.

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

Add *.precompiled.js as per
https://stackoverflow.com/questions/20314796/which-files-are-generated-when-executing-dart2js-and-why

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

Line 1 somehow different from upstream

Added directory for dartdoc tool. Disambiguated comment for pubspec.lock
pattern.

As of Dart 1.12, the pub package manager will start generating a .packages file that will eventually replace the packages/ directory.

Dart2js (Dart's JavaScript compiler) no longer creates `foo.dart.precompiled.js` files when compiling.

add the `.pub/` directory, and change the `packages` filter to match sub-folders.

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.