BluetoothAdvertisingEventInit.{manufacturer,service}Data's type is unclear

[rakuco]

The BluetoothAdvertisingEventInit dictionary has

  BluetoothManufacturerDataMap manufacturerData;
  BluetoothServiceDataMap serviceData;

whose handling is unclear:

• The BluetoothManufacturerDataMap and BluetoothServiceDataMap interfaces do not have constructors, so user code cannot create new instances at will.
• Even if they did, the interfaces do not expose any methods to add or remove data from each map: not only are the maplike declarations readonly, but the backing "map entries" is not exposed to user code.
• https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothadvertisingevent-bluetoothadvertisingevent performs some data conversions while looping through manufacturerData and serviceData that look superfluous/odd if the input data is of the same type as the output.

[reillyeon]

This dictionary would only be used when script is constructing a BluetoothAdvertisingEvent, which is not currently supported in Chromium (only UA-constructed objects are implemented) so I'm not surprised the specification text is a little wonky.