call Map#set only when needed

lib/single-file/doc-helper.js

@@ -178,9 +178,9 @@ this.docHelper = this.docHelper || (() => {
 						}
 					}
 				}
+				cacheElementsHidden.set(element, hidden);
 			}
-			hidden = Boolean(hidden);
-			cacheElementsHidden.set(element, hidden);
+			hidden = Boolean(hidden);			
 		}
 		return hidden;
 	}