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.