Proposal for the use of interfaces to allow mocking of final classes like Stage

DisplayObject.as

package flash.display.DisplayObject
{
	public class DisplayObject extends EventDispatcher implements IDisplayObject
	{
		public function DisplayObject()
		{
		}

		public function get stage():IStage
		{
			// return Stage by default
		}

		// ...other interface methods...
	}
}

InvalidatingSprite.as

package
{
	public class InvalidatingSprite extends Sprite
	{
		protected var isInvalidating:Boolean;

		public function InvalidatingSprite()
		{
		}

		public function invalidateDisplayList()
		{
			if (stage && !isInvalidating)
			{
				isInvalidating = true;

				addEventListener(Event.RENDER, renderHandler);
				addEventListener(Event.ENTERFRAME, enterframeHandler);

				stage.invalidate();
			}
		}

		// ...other invalidation code...
	}
}

InvalidatingSpriteTest.as

package
{
	public class InvalidatingSpriteTest
	{
		public var sprite:InvalidatingSprite;

		public function InvalidatingSpriteTest()
		{
		}

		[Before]
		public function setUp():void
		{
			sprite = new InvalidatingSprite();
		}

		[After]
		public function tearDown():void
		{
			sprite = null;
		}

		[Test]
		public function sprite_invalidates_stage():void
		{
			var mockStage:MockStage = new MockStage();
			sprite.stage = mockStage;

			sprite.invalidateDisplayList();

			assertThat(mockStage.hasInvalidated);
		}
	}
}

MockStage.as

package
{
	public class MockStage implements IStage
	{
		public var hasInvalidated:Boolean;

		public function MockStage()
		{
		}

		public function invalidate():void
		{
			hasInvalidated = true;
		}

		// ...other interface methods...
	}
}