src/vs/workbench/common/editor.ts/SaveSourceFactory/registerSource
class SaveSourceFactory:	registerSource(id: string, label: string): SaveSource {
		let sourceDescriptor = this.mapIdToSaveSource.get(id);
		if (!sourceDescriptor) {
			sourceDescriptor = { source: id, label };
			this.mapIdToSaveSource.set(id, sourceDescriptor);
		}

		return sourceDescriptor.source;
	}