Mouse: Update elIsCancel to use composedPath instead of event.target

When using web-components with shadowRoot, the mouseDown event is triggered with the event.target pointing to the entire web component, not inner nodes that were clicked, thus to cancel the mouseDown based on inner-elements we have to use composedPath instead of event.target

ui/widgets/mouse.js

@@ -94,7 +94,9 @@ return $.widget( "ui.mouse", {
 
 		this._mouseDownEvent = event;
 
-		let [ first = event.target ] = 'composedPath' in event.originalEvent && event.originalEvent.composedPath() || []
+		var first = (
+			"composedPath" in event.originalEvent ? event.originalEvent.composedPath() : [ ]
+		)[ 0 ] || event.target;
 
 		var that = this,
 			btnIsLeft = ( event.which === 1 ),