Merge pull request #87 from fzhangtj/master

fix assertion issue

Runtime/material/input_decorator.cs

         protected override void removeChildRenderObject(RenderObject child) {
             D.assert(child is RenderBox);
             D.assert(this.renderObject.childToSlot.ContainsKey((RenderBox) child));
+            var slot = this.renderObject.childToSlot[(RenderBox) child];
-            D.assert(!this.renderObject.slotToChild.ContainsKey((_DecorationSlot) this.slot));
+            D.assert(!this.renderObject.slotToChild.ContainsKey(slot));
         }
 
         protected override void moveChildRenderObject(RenderObject child, object slotValue) {