perf: replace lookbehind regex in _normalizeLineEndings with string replaceAll

CHANGELOG.md

@@ -21,6 +21,7 @@
 * Fix: `UsernameType.emailLocalPart` returns the local part; secure random Message-IDs; exact plus-alias matching; a working DKIM signer.
 * Fix: a reply that was received only partially when a connection was lost no longer corrupts the parsing of the replies after a reconnect.
 * Fix: non-ASCII display names are rendered as bare RFC 2047 encoded-words restricted to the characters allowed in a phrase instead of being placed inside a quoted-string; encoded words are labelled `UTF-8`.
+* Fix: replace the slow lookbehind regex in `TextMimeData` line ending normalisation with plain string replacements.
 # 2.2.3
 * Chore: Shorten package description to 60-180 characters for pub.dev score
 * Docs: Add example file (example/example.dart)

lib/src/mime_data.dart

@@ -133,7 +133,7 @@ class TextMimeData extends MimeData {
 
   /// Normalizes bare LF to CRLF for RFC 5322 compliance.
   static String _normalizeLineEndings(String text) =>
-      text.replaceAll(RegExp(r'(?<!\r)\n'), '\r\n');
+      text.replaceAll('\r\n', '\n').replaceAll('\n', '\r\n');
 
   /// Splits a multipart [body] at its [boundary] delimiters.
   ///