Flash 8.
symbolInstance.cacheAsBitmap
Property; a Boolean value that specifies whether run-time bitmap caching is enabled.
Note: Starting w/ Flash Professional CS5.5, users should switch to using the "symbolInstance.bitmapRenderMode" property instead of this property.
The following example enables run-time bitmap caching for the first element in the first frame on the first layer:
fl.getDocumentDOM().getTimeline().layers[0].frames[0].elements[0].cacheAsBitmap = true;