[fuchsia] Add missing vm:entry-point annotation

Change-Id: I42be70aba4aa5ed451fcc4afca02f0830f8d82d7
Reviewed-on: https://dart-review.googlesource.com/76220
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Zach Anderson <zra@google.com>
diff --git a/sdk/lib/io/stdio.dart b/sdk/lib/io/stdio.dart
index e9d3862..241e7d2 100644
--- a/sdk/lib/io/stdio.dart
+++ b/sdk/lib/io/stdio.dart
@@ -396,7 +396,7 @@
 int _stdoutFD = 1;
 int _stderrFD = 2;
 
-// This is an embedder entrypoint.
+@pragma('vm:entry-point')
 void _setStdioFDs(int stdin, int stdout, int stderr) {
   _stdinFD = stdin;
   _stdoutFD = stdout;