How can i draw a shape, that is only rounded at it's bottom, without using flex libraries?
var _myShape:Shape = new Shape();
_myShape.graphics.lineStyle(4,0x000000,1,true,....);
_myShape.graphics.drawRoundRect(0,0,50,50,10);
if you don't want to use Flex libraries (as you've commented on Glenn's answer) and if you're only concerned will fills, you could employ a masking technique on your sprites.
var sprite:Sprite = new Sprite();
sprite.graphics.beginFill(0xFF0000, 1.0);
sprite.graphics.drawRoundRect(0, 0, 300, 200, 100, 100);
sprite.graphics.endFill();
var spriteMask:Shape = new Shape();
spriteMask.graphics.beginFill(0);
spriteMask.graphics.drawRect(0, sprite.height / 2, sprite.width, sprite.height / 2);
spriteMask.graphics.endFill();
sprite.mask = spriteMask;
sprite.addChild(spriteMask);
addChild(sprite);
including strokes is a little trickier, but still possible.