use Map#size to get the size of the Map

lib/single-file/single-file-core.js

@@ -286,7 +286,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 		}
 
 		getMaxResources() {
-			return Array.from(this.requests.keys()).length;
+			return this.requests.size;
 		}
 
 		async run(onloadListener, options) {