[Library] Change signature of dart:io exit to return Never

(see issue - #42006)

Bug:42006
Change-Id: I72b474c6ea398f4e7bd746beac650e625be8857c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148920
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

CHANGELOG.md

@@ -4,6 +4,13 @@
 
 ### Core libraries
 
+#### `dart:io`
+
+*   [#42006][]: The signature of `exit` has been changed to return the
+    `Never`type instead of `void`. since no code will run after it,
+
+[#42006]: https://github.com/dart-lang/sdk/issues/42006
+
 #### `dart:convert`
 
 *   **Breaking Change** [#41100][]: When encoding a string containing unpaired

sdk_nnbd/lib/_internal/js_dev_runtime/patch/io_patch.dart

@@ -303,7 +303,7 @@ class _Platform {
 @patch
 class _ProcessUtils {
   @patch
-  static void _exit(int status) {
+  static Never _exit(int status) {
     throw UnsupportedError("ProcessUtils._exit");
   }
 

sdk_nnbd/lib/_internal/js_runtime/lib/io_patch.dart

@@ -303,7 +303,7 @@ class _Platform {
 @patch
 class _ProcessUtils {
   @patch
-  static void _exit(int status) {
+  static Never _exit(int status) {
     throw new UnsupportedError("ProcessUtils._exit");
   }
 

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

@@ -135,7 +135,7 @@ Function _getWatchSignalInternal() => _ProcessUtils._watchSignalInternal;
 @patch
 class _ProcessUtils {
   @patch
-  static void _exit(int status) native "Process_Exit";
+  static Never _exit(int status) native "Process_Exit";
   @patch
   static void _setExitCode(int status) native "Process_SetExitCode";
   @patch

sdk_nnbd/lib/io/process.dart

@@ -7,7 +7,7 @@ part of dart.io;
 // TODO(ager): The only reason for this class is that we
 // cannot patch a top-level at this point.
 class _ProcessUtils {
-  external static void _exit(int status);
+  external static Never _exit(int status);
   external static void _setExitCode(int status);
   external static int _getExitCode();
   external static void _sleep(int millis);
@@ -49,7 +49,7 @@ class _ProcessUtils {
  * program to the surrounding environment. This will avoid any
  * cross-platform issues.
  */
-void exit(int code) {
+Never exit(int code) {
   ArgumentError.checkNotNull(code, "code");
   if (!_EmbedderConfig._mayExit) {
     throw new UnsupportedError(