Updates based on discussions with Dart team

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