问第2? 1046行:类型未找到或不是编译时常量: shield

EN

import flash.display.MovieClip;
import flash.events.Event;
import flash.ui.Keyboard;

public class Ship extends MovieClip{

    var velocity:Number;
    var shootLimiter:Number;
    var health:Number;
    var maxHealth:Number; 

        function Ship(){

        velocity = 10;
        shootLimiter = 0;
        health = 100;
        maxHealth = 100;
        addEventListener("enterFrame", move);

    }

    function kill(){

        var explosion = new Explosion();
        stage.addChild(explosion);
        explosion.x = this.x;
        explosion.y = this.y;

        removeEventListener("enterFrame", move);
        this.visible = false;

        Game.gameOver();

    }

    function takeDamage(d){

        health -= d;

        if(health<=0){
            health = 0;
            kill();
        }

        Game.healthMeter.bar.scaleX = health/maxHealth;

    }

    function move(e:Event){

        shootLimiter += 1;

        if(Key.isDown(Keyboard.D)){
            this.x = this.x + velocity;
        }
        if(Key.isDown(Keyboard.A)){
            this.x = this.x - velocity;
        }
        if(Key.isDown(Keyboard.W)){
            this.y = this.y - velocity;
        }
        if(Key.isDown(Keyboard.S)){
            this.y = this.y + velocity;
        }
        if(Key.isDown(Keyboard.SPACE) && shootLimiter > 8){
            shootLimiter = 0;
            var b = new Bullet();
            stage.addChild(b);
            b.x = this.x + 50;
            b.y = this.y + 3;
        }

        if(shield.visible == true){
            shield.alpha -= 0.0005;
            if(shield.alpha == 0){
                shield.visible = false;
                shield.alpha = 1;
            }
        }

    } 

}