Revert "Remove vmservice_io.main from entry points. (#5625)" (#5711)

This caused failures in Flutter microbenchmarks.

See: flutter/flutter#19096 and flutter/flutter#19210

This reverts commit 4466d61a.

runtime/dart_vm_entry_points.txt

@@ -43,3 +43,4 @@ dart:ui,SemanticsUpdate,SemanticsUpdate._
 dart:ui,SemanticsUpdateBuilder,SemanticsUpdateBuilder.
 dart:ui,Shader,Shader._
 dart:ui,TextBox,TextBox._
+dart:vmservice_io,::,main