[Bug Report] upload component uses the drag method errors and cannot achieve the normal effect

### Element UI version
0.1.0

### OS/Browsers version
WIN10 Chrome

### Vue version
3.0.0-alpha.11

### Reproduction Link
https://codepen.io/Stories609449489/pen/KKdPxQm?editable=true

### Steps to reproduce
The example on the official website is now like this:

event.js:510 Uncaught TypeError: this._drop is not a function
at Object.handleEvent (event.js:510)

### What is Expected?
normal effect

### What is actually happening?
the upload component uses the drag and drop method to report an error, and at the same time, it cannot be triggered before-upload event when uploading using the drag and drop method.