Update Variables and Local Variable declaration according to spec

[sgrekhov]

Variables and Local Variable declaration sections of the specification were updated. See dart-lang/language@af27cfb. Need to update co19 tests accordingly

[eernstg]

Right, these updates are quite substantial. The changes are the null safety related, so there should not be anything new for tests that are already targeting the language at a version where null safety has been introduced.

However, it would certainly be valuable to get an extra pair of eyes on the specification and the actual tool behavior, such that we can detect ways in which they disagree, if any. ;-)