The engine doesn't support/expose it anymore but you can still do it.
import flash.display.Shape;
...
private var canvas:Shape = new Shape();
...
canvas.graphics.lineStyle(1.0, 0xFF0000);
canvas.graphics.moveTo(10, 10);
canvas.graphics.lineTo(20, 20);
canvas.graphics.curveTo(100, 100, 50, 10);
canvas.graphics.endFill();
...
FlxG.buffer.draw(canvas);