Missing follow-up event for 'sourcedataloading' event

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**mapbox-gl-js version**: 0.44.1

### Steps to Trigger Behavior

1. Open https://jsfiddle.net/Scarysize/2nq2v2an/3/ and open browser console
2. Watch for `start loading: source-x` logs
3. Watch for `done loading: source-x` logs
4. Additionally I'm logging `isSourceLoaded()` results every 3 seconds

### Expected Behavior

For every `(source-)dataloading` event of a given source, there should be a `(source-)data` event with `isSourceLoaded: true`.

The documentation states:

> All sourcedataloading events are followed by a sourcedata or error event.

### Actual Behavior

The `sourcedata` event is only dispatched for one of the four souces in the example.

EDIT: If I manually check if the sources are loaded (map.isSourceLoaded(...)) the expected value is returned. I'm just missing the event.

### Context

We are trying to incrementally add source & layer pairs. So when one source-layer-combination is done "loading", we would like to add the next pair. Basically buffering future sources.

Start with the linked JSFiddle reproduction and the sourcedataloading, sourcedata, and error event behavior described in the issue. Compare those events with repeated map.isSourceLoaded() checks; done means each relevant loading event has the documented follow-up event or an error, including the multiple-source case.

Written by the indexing model from the issue text.