[io/win] Ensure console window is not created when running in windows…

… mode.

Fixes #39945

TEST=ci, manually check flutter windows app doesn't pop-up console.

Change-Id: I05983947de10058e4b0d2f9cbe795bd52f97ad05
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237400
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

runtime/bin/process_win.cc

@@ -553,6 +553,13 @@ class ProcessStarter {
         EXTENDED_STARTUPINFO_PRESENT | CREATE_UNICODE_ENVIRONMENT;
     if (!Process::ModeIsAttached(mode_)) {
       creation_flags |= DETACHED_PROCESS;
+    } else {
+      // Ensure that if console needs to be created, it is created hidden.
+      // Normally stdout for console dart application is associated with console
+      // that is launched from, but for gui applications(flutter on windows)
+      // console might be absent, will be created by CreateProcessW below.
+      // When that happens we ensure that console window doesn't pop up.
+      creation_flags |= CREATE_NO_WINDOW;
     }
     BOOL result = CreateProcessW(
         NULL,  // ApplicationName