Got verbal LGTM from @vsavkin

Merging PR #4198 on behalf of @yjbanov to branch presubmit-yjbanov-pr-4198.

Merging PR #4198 on behalf of @yjbanov to branch presubmit-yjbanov-pr-4198.

Merging PR #4198 on behalf of @yjbanov to branch presubmit-yjbanov-pr-4198.

You say "This PR alone does not fix the issue".
I just discovered that you can get this to work before I fix dart-lang/sdk#24355

bool checkedMode() {
  var k = false;
  assert((k = true));
  return k;
}

main() {
  if (checkedMode()) print('Hello');
  if (checkedMode()) print('Bye');
}

The difference is the bool vs var at the declaration of k.
Actually having a type makes the code look 'bigger', just enough to make the code look too big to inline.

We should fix that: dart-lang/sdk#24365

@rakudrama we still need dart-lang/sdk#24354 though, no?

@yjbanov Try it. Most likely you will see some benefit. dart-lang/sdk#24354 might take a long time to fix.

@rakudrama I tried hard-coding return false and while it does inline the value at call sites, it fails to propagate it, exactly as described in dart-lang/sdk#24354

@yjbanov I guess I misunderstood how these issues are connected.

I understand now that you call checkedMode() once and pass the value everywhere.
Even if we do an excellent job of improving dart-lang/sdk#24354, that pattern will always be a bit fragile, if there is one step that we cannot get you lose the whole tree under that step.

If you want to experiment, try passing null for disabled and true for enabled.
We track null/non-null more accurately than any other fact.
If null/true does not work, then false/true will not work either.

Otherwise you could change from:

• passing down one result from checkedMode() into a tree of code

to:

• Calling checkedMode() at each place.

You could also pass a singleton object with a checkedMode() method as above. We would inline that if we can prove the type of the singleton (we should be able to, again, if this fails, so will false/true).

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