Fix type annotation #134

src/polyfill/Blob.js

    * @param  {string} contentType Optional, content type of new Blob object
    * @return {Blob}
    */
-  slice(start:?number, end:?number, contentType='':?string):Blob {
+  slice(start:?number, end:?number, contentType:?string=''):Blob {
     if(this._closed)
       throw 'Blob has been released.'
     log.verbose('slice called', start, end, contentType)