2nd list for local copies of messages stored on server side
GlobalStorage gets/posts messages from/to server
Use GlobalStorage on button 'Sync local to global'
Visual feedback when error on posting messages
Server-side Dart script stores messages in json file

You need to have Dart support on the server to
use this script. I am pretty sure this does not work
with dart2js and a JavaScript vm.
Do not commit global message store (global.json)

It is global for one instance of this application.
Support clearing of global messages

Client:
  Method clear() in GlobalStorags posts 'IDLE'	to global.dart.
  Mime-Type is 'text/plain' so it is a command, not a message.
  Now there are two methods that post to global.dart so internal
  method was added to avoid duplicate code.
  It will be necessary to get global.json while application is
  already running for some time, therefore a new method get() added.
Server:
  When new request, its Content-Type is checked first to distinguish
  between a command and new messages.
  On CLEAR command 'global.json' gets deleted.
  TODO:
    Post content list will grow to infinity, but I'm pretty sure
    before something ugly happens.