[infra] Support snippets with omissions #3688

From #3641 (comment)

It would be cool if we could have snippets where we can omit things

// snippet-start#dangerous
Future<void> dangerousCall() async {
  // Do x-y-z.
  // snippet-omit-start
  // 1. Get the Activity (e.g., Activity "A")
  final activity = androidActivity(engineId);

  // 2. An `await` occurs. The main thread is freed.
  //    While waiting, Android might destroy Activity "A" and create "B".
  await someOtherFuture();

  // 3. CRASH: The code resumes, but `activity` is now a stale
  //    reference to the destroyed Activity "A".
  if (activity != null) {
    someGeneratedApi.doSomething(activity); // This will crash
    activity.release();
  }
  // snippet-omit-end
}
// snippet-end#dangerous

Which would then become

Future<void> dangerousCall() async {
  // Do x-y-z.
  // [...]
}