Now that https://github.com/dart-lang/sdk/issues/41985 is fixed, the
analyzer detects 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.

Note that one assertion is removed by this PR (`heightFactor != null
|| (heightFactor == 1.0 && heightDelta == 0.0)`).  This is because
`heightFactor == 1.0 && heightDelta == 0.0` can only be true if
`heightFactor != null`, so this assertion is equivalent to simply
asserting that `heightFactor != null`, which is already asserted two
lines above.