Obtain global tag prefix in one pass

Author: kekavc24

pkgs/yaml/lib/src/scanner.dart

@@ -937,11 +937,9 @@ class Scanner {
     /// Both tag uri and local tags can be used as prefixes.
     ///
     /// See: https://yaml.org/spec/1.2.2/#6822-tag-prefixes
-    var prefix = _scanner.peekChar() == EXCLAMATION
+    final prefix = _scanner.peekChar() == EXCLAMATION
         ? _scanTagHandle(directive: true, isGlobalTagPrefix: true).tagHandle
-        : '';
-
-    prefix += _scanTagUri(); // Readability's sake
+        : _scanTagUri();
 
     if (prefix.isEmpty) {
       throw YamlException(