src/shared/index.js/onDev
export function onDev ( eventName, handler ) {
	if ( eventName === 'teardown' ) {
		console.warn( "Use component.on('destroy', ...) instead of component.on('teardown', ...) which has been deprecated and will be unsupported in Svelte 2" );
		return this.on( 'destroy', handler );
	}

	return on.call( this, eventName, handler );
}