[nnbd] Fix Process.stderr providing stdout instead in NNBD dart:io.

Bug: dart-lang/sdk#40040
Change-Id: Ia318180de1b1614502671c6bf4051d79ae7808f3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134327
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>

sdk_nnbd/lib/_internal/vm/bin/process_patch.dart

@@ -507,12 +507,12 @@ class _ProcessImpl extends _ProcessImplNativeWrapper implements Process {
       _NativeSocket exitHandler) native "Process_Wait";
 
   Stream<List<int>> get stdout =>
-      _stdout ?? (throw StateError("Process is detached"));
+      _stdout ?? (throw StateError("stdio is not connected"));
 
   Stream<List<int>> get stderr =>
-      _stdout ?? (throw StateError("Process is detached"));
+      _stderr ?? (throw StateError("stdio is not connected"));
 
-  IOSink get stdin => _stdin ?? (throw StateError("Process is detached"));
+  IOSink get stdin => _stdin ?? (throw StateError("stdio is not connected"));
 
   Future<int> get exitCode =>
       _exitCode?.future ?? (throw StateError("Process is detached"));