prevent subclassing by making public constructor a factory

pkgs/io/lib/src/shared_stdin.dart

diff --git a/pkgs/io/lib/src/shared_stdin.dart b/pkgs/io/lib/src/shared_stdin.dart
index 9b783d9..3bb0d27 100644
--- a/pkgs/io/lib/src/shared_stdin.dart
+++ b/pkgs/io/lib/src/shared_stdin.dart
@@ -35,7 +35,10 @@
   ///
   /// Calling this constructor more than once with the same source [stream]
   /// will likely result in an error.
-  SharedStdIn(Stream<List<int>> stream) {
+  factory SharedStdIn(Stream<List<int>> stream) => SharedStdIn._(stream);
+
+  /// Actual constructor is private to prevent subclassing.
+  SharedStdIn._(Stream<List<int>> stream) {
     _sub = stream.listen(_onInput);
   }