[js] make __interfLoop oh so smart (closes #7834)

std/js/Boot.hx

@@ -161,13 +161,14 @@ class Boot {
 			return false;
 		if( cc == cl )
 			return true;
-		var intf : Dynamic = cc.__interfaces__;
-		if( intf != null )
+		if( js.Object.prototype.hasOwnProperty.call(cc, "__interfaces__") ) {
+			var intf : Dynamic = cc.__interfaces__;
 			for( i in 0...intf.length ) {
 				var i : Dynamic = intf[i];
 				if( i == cl || __interfLoop(i,cl) )
 					return true;
 			}
+		}
 		return __interfLoop(cc.__super__,cl);
 	}