fix(flowDragEnter): wrong triggering of flow-drag-leave

You can reproduce the issue here: http://flowjs.github.io/ng-flow/
Drag a file into the browser. The drop area will become green. Now don't drop the file, but drag it over some links and other elements to generate `dragleave` events. You'll notice that the drop area isn't green all the time, it blinks.

My browser (just in case): Chrome 37.0.2062.103 m, Windows 7 x64