[udaaff]

Копипаст из хэлпа с небольшой модификацией:

import flash.geom.*;
var fillType:String = "linear"
var colors:Array = [0xFF0000, 0x0000FF];
var alphas:Array = [100, 100];
var ratios:Array = [0x00, 0xFF];
var matrix:Matrix = new Matrix();
matrix.createGradientBox(20, 20, Math.PI / 2, 0, 0);
var spreadMethod:String = "pad";
this.beginGradientFill(fillType, colors, alphas, ratios, matrix, spreadMethod); 
this.moveTo(0, 0);
this.lineTo(0, 100);
this.lineTo(100, 100);
this.lineTo(100, 0);
this.lineTo(0, 0);
this.endFill();