[dart2js]: expose mapping between deferred imports and load-ids

[sigmundch]

The current deferred-map exposes is missing an important piece of information that tools need to properly configure deferred loading bundling: how to map deferred-imports to load-ids.

An import is identified as a pair (locationUri, prefix) whereas a load-id is just a string. Today the load-id string can be a simple numeric sequence (if using --simple-load-ids) or a name closely resembling the prefix (with a suffix to guarnatee uniqueness).

The deferred map today is structured as a map: locationUri -> load-id -> loadlists. When users don't use --simple-load-ids, they can reverse-engineer what was the corresponding prefix. Instead, we'd like to add the prefix as part of the data being exposed (either in the deferred-map or in a separate file).