Obtain global tag prefix in one pass

Author: kekavc24

pkgs/yaml/lib/src/scanner.dart

@@ -934,21 +934,28 @@ class Scanner {
 
     _skipBlanks();
 
+    var prefix = '';
+
     /// 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
-        ? _scanTagHandle(directive: true, isGlobalTagPrefix: true).tagHandle
-        : '';
+    if (_scanner.peekChar() == EXCLAMATION) {
+      prefix = _scanTagHandle(
+        directive: true,
+        isGlobalTagPrefix: true,
+      ).tagHandle;
+    } else {
+      prefix = _scanTagUri();
 
-    prefix += _scanTagUri(); // Readability's sake
+      if (prefix.isEmpty) {
+        throw YamlException(
+          'Expected a non-empty global tag prefix',
+          _scanner.emptySpan,
+        );
+      }
+    }
 
-    if (prefix.isEmpty) {
-      throw YamlException(
-        'Expected a non-empty global tag prefix',
-        _scanner.emptySpan,
-      );
-    } else if (!_isBlankOrEnd) {
+    if (!_isBlankOrEnd) {
       throw YamlException('Expected whitespace.', _scanner.emptySpan);
     }