Fixes #98435

Using exec from bash causes the new process to replace the current one (the bash script), so that any signals sent from the host be sent to the new process (the dart process). Before this change, the test I added will leak the dart process.

It does not seem like there is an exec equivalent on Windows https://stackoverflow.com/questions/20892102/kill-batch-file-in-such-a-way-that-its-children-are-killed-too-in-windows

Context

The following quotes are from the bash manpages:

exec [-cl] [-a name] [command [arguments]]

If command is specified, it replaces the shell. No new process is created. The arguments become the arguments to command. If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what login(1) does. The -c option causes command to be executed with an empty environment. If -a is supplied, the shell passes name as the zeroth argument to the executed command. If command cannot be executed for some reason, a non-interactive shell exits, unless the shell option execfail is enabled, in which case it returns failure. An interactive shell returns failure if the file cannot be executed. If command is not specified, any redirections take effect in the current shell, and the return status is 0. If there is a redirection error, the return status is 1.