Shared IndexedDB store between different web browsers
Browsers can't access each other's data directly, but, given your diagram, it looks like IndexedDB is on the shared server and not in the particular browser so I can't see the problem.
If you have your database running on local server and it is accessible only with Dartium and for some reason you don't want or can't compile the browser part to JS(with dart2js) or do something else to make it accessible from the browsers without DartVM. then I can think of 3 ways how to perform browser to browser data transfer without the server(on which application is hosted):
- With HTML5 Drag and Drop between browser windows.
- Creating WebRTC client\server and transferring the data this way.
- Using remote server and simple authentication with Gmail,Facebook etc. [the reasonable way]