Added Phaser.Filter and started moving the shaders over into their own filter classes, so they won't all get bundled in unless needed.

src/gameobjects/Sprite.js

@@ -118,11 +118,16 @@ Phaser.Sprite = function (game, x, y, key, frame) {
     }
     else
     {
-        if (key == null || this.game.cache.checkImageKey(key) == false)
+        if (key === null || typeof key === 'undefined')
         {
             key = '__default';
             this.key = key;
         }
+        else if (typeof key === 'string' && this.game.cache.checkImageKey(key) == false)
+        {
+            key = '__missing';
+            this.key = key;
+        }
 
         PIXI.Sprite.call(this, PIXI.TextureCache[key]);