loadSkin( skinIndex ) {

    		const skinDef = this.json.skins[ skinIndex ];

    		const skinEntry = { joints: skinDef.joints };

    		if ( skinDef.inverseBindMatrices === undefined ) {

    			return Promise.resolve( skinEntry );

    		}

    		return this.getDependency( 'accessor', skinDef.inverseBindMatrices ).then( function ( accessor ) {

    			skinEntry.inverseBindMatrices = accessor;

    			return skinEntry;

    		} );

    	}
Specification: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#skins
@param {number} skinIndex
@return {Promise<Object>}