Fix links

[kwalrath]

I found these 404s through https://validator.w3.org/checklink?uri=https%3A%2F%2Fwww.dartlang.org%2F&hide_redirects=on&hide_type=all&recursive=on&depth=2&check=Check#results1:

• https://www.dartlang.org/dart-by-example/ -> https://www.dartlang.org/docs/serverguide.html
• https://www.dartlang.org/articles/zones/ -> https://api.dartlang.org/apidocs/channels/stable/#dart-async.ZoneDelegate (and other stable/# links)
• https://www.dartlang.org/articles/json-web-service/ -> https://dart-lang.github.com/js-interop/docs/js.html redirected to http://dart-lang.github.io/js-interop/docs/js.html
• https://www.dartlang.org/community/who-uses-dart.html -> http://blogs.adobe.com/flashpro/2013/05/16/toolkit-for-dart-flash-pro/ redirected to http://blogs.adobe.com/animate/2013/05/16/toolkit-for-dart-flash-pro/ (should be http://blogs.adobe.com/animate/toolkit-for-dart-flash-pro/)
• https://www.dartlang.org/docs/tutorials/shared-pkgs/ -> https://www.dartlang.org/docs/tutorials/shared-pkgs/futures/
• https://www.dartlang.org/docs/tutorials/httpserver/ -> https://api.dartlang.org/apidocs/channels/stable/

False 404s include:

• https://www.dartlang.org/articles/serialization/ -> https://google.com/+EricSchneller
• https://www.dartlang.org/docs/ -> https://drone.io/ (which http://drone.io/ redirects to)

[Sfshaza]

Closing. We have a link checker for the new sites.