function ShellTestEnvironment() {
        this.name_counter = 0;
        this.all_loaded = false;
        this.on_loaded_callback = null;
        Promise.resolve().then(function() {
            this.all_loaded = true;
            if (this.on_loaded_callback) {
                this.on_loaded_callback();
            }
        }.bind(this));
        this.message_list = [];
        this.message_ports = [];
    }
Signal to the test environment that the tests are ready and the on-loaded
callback should be run.

Shadow realms are not *really* a DOM context: they have no `onload` or similar
event for us to use to set up the test environment; so, instead, this method
is manually triggered from the incubating realm

@param {Function} message_destination - a function that receives JSON-serializable
data to send to the incubating realm, in the same format as used by RemoteContext