[senocular]

In ActionScript 2, you could use Key.isDown(keyCode) to determine whether or not a key was being pressed on the keyboard. This command is no longer available in ActionScript 3. In fact, there is no Key object in ActionScript 3. You can still access keyCodes by name, but through the Keyboard (flash.ui.Keyboard) class. For key recognition in ActionScript 3, you have to use keyboard events or, more specifically the KeyboardEvent.KEY_DOWN and KeyboardEvent.KEY_UP events. When a key is pressed on the keyboard, the KeyboardEvent.KEY_DOWN is dispatched and you can tell which key was pressed by checking KeyboardEvent.keyCode in the event handler. Same applies to keys being released in the KeyboardEvent.KEY_UP.

Using these events, you could recreate the functionality provided by the Key class in ActionScript 2. However, you should know that the KeyboardEvents only work when the Flash player has focus. This means a key could be pressed in Flash and released outside of Flash without Flash knowing that it was released (not having received the KEY_UP event). As a result, you should assume all keys released upon deactivation (loss of focus) of the Flash player. The following recreation of the Key class takes that into consideration.

// Key.as
package {
    
    import flash.display.Stage;
    import flash.events.Event;
    import flash.events.KeyboardEvent;
    
    /**
     * The Key class recreates functionality of
     * Key.isDown of ActionScript 1 and 2. Before using
     * Key.isDown, you first need to initialize the
     * Key class with a reference to the stage using
     * its Key.initialize() method. For key
     * codes use the flash.ui.Keyboard class.
     *
     * Usage:
     * Key.initialize(stage);
     * if (Key.isDown(Keyboard.LEFT)) {
     *    // Left key is being pressed
     * }
     */
    public class Key {
        
        private static var initialized:Boolean = false;  // marks whether or not the class has been initialized
        private static var keysDown:Object = new Object();  // stores key codes of all keys pressed
        
        /**
         * Initializes the key class creating assigning event
         * handlers to capture necessary key events from the stage
         */
        public static function initialize(stage:Stage) {
            if (!initialized) {
                // assign listeners for key presses and deactivation of the player
                stage.addEventListener(KeyboardEvent.KEY_DOWN, keyPressed);
                stage.addEventListener(KeyboardEvent.KEY_UP, keyReleased);
                stage.addEventListener(Event.DEACTIVATE, clearKeys);
                
                // mark initialization as true so redundant
                // calls do not reassign the event handlers
                initialized = true;
            }
        }
        
        /**
         * Returns true or false if the key represented by the
         * keyCode passed is being pressed
         */
        public static function isDown(keyCode:uint):Boolean {
            if (!initialized) {
                // throw an error if isDown is used
                // prior to Key class initialization
                throw new Error("Key class has yet been initialized.");
            }
            return Boolean(keyCode in keysDown);
        }
        
        /**
         * Event handler for capturing keys being pressed
         */
        private static function keyPressed(event:KeyboardEvent):void {
            // create a property in keysDown with the name of the keyCode
            keysDown[event.keyCode] = true;
        }
        
        /**
         * Event handler for capturing keys being released
         */
        private static function keyReleased(event:KeyboardEvent):void {
            if (event.keyCode in keysDown) {
                // delete the property in keysDown if it exists
                delete keysDown[event.keyCode];
            }
        }
        
        /**
         * Event handler for Flash Player deactivation
         */
        private static function clearKeys(event:Event):void {
            // clear all keys in keysDown since the player cannot
            // detect keys being pressed or released when not focused
            keysDown = new Object();
        }
    }
}