[flow analysis] Update the spec to match implementation of ==.

[stereotype441]

In fully sound null safety mode, flow analysis should know that a test like expr == null is guaranteed to evaluate to false if the static type of expr is non-nullable. But in unsound null safety mode, no such guarantee can be made.

Since support for unsound null safety was only recently removed from the CFE (see
dart-lang/sdk@0060b0f), flow analysis still conservatively assumes that an expression with a non-nullable static type might, nonetheless, still be null.

This change updates the spec to match the implementation in this regard, and adds a reference to
#3100, where we are discussing the possibility of changing the behavior.

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[eernstg]

LGTM.

It'll be nice when we can go back to the old rule in the spec and the implementation, though. ;-)