Ignore dead_code hints for weak-only null checks. (#63007)

* Ignore dead_code hints for weak-only null checks.

When https://github.com/dart-lang/sdk/issues/41985 is fixed, the
analyzer will begin detecting dead code due to "unnecessary" null
checks.  Since we want to retain null checks in Flutter even when they
appear unnecessary (in order to preserve runtime behavior in weak
mode), we need to suppress these dead code hints.

* Add comments explaining why we're ignoring dead_code hints