Flash MX 2004. Value "bitmap" added in Flash CS4 Professional.
fill.style
Property; a string that specifies the fill style. Acceptable values are "bitmap", "solid", "linearGradient", "radialGradient", and "noFill". If this value is "linearGradient" or "radialGradient", the fill.colorArray and fill.posArray properties are also available. If this value is "bitmap", the fill.bitmapIsClipped and fill.bitmapPath properties are also available.
The following example specifies the colors to use in a linear gradient for the current selection:
var fill = fl.getDocumentDOM().getCustomFill();
fill.style= "linearGradient";
fill.colorArray = [ 0x00ff00, 0xff0000, 0x0000ff ];
fill.posArray= [0,100, 200];
fl.getDocumentDOM().setCustomFill( fill );