code: function onprogress( data ) {
  	debug( 'got "progress" event: %o', data );
  	const xhr = requests[ data.callbackId ];
  	if ( xhr ) {
  		const prog = new ProgressEvent( 'progress', data );
  		const target = data.upload ? xhr.upload : xhr;
  		target.dispatchEvent( prog );
  	}
  }
docs: Handles a "progress" event being proxied back from the iframe page.
  
  @param {Object} data
  @private