Most of these are harmless, and are best fixed simply by removing the
redundant check or assignment. The one in FlowType.compareBlocks,
however, revealed a real problem. This code checks for nullness of
`a` *after* having called a method on `a`. Assuming that `a` can
indeed be `null` here, the check must come first to avoid a
`NullPointerException`.
In several places, I saw code of this form:
if (thing == null)
assert thing != null : ... ;
I honestly don't understand the purpose of that `if` statement. Why
not just have the `assert` statement there directly? I removed the
seemingly irrelevant `if` statements in these cases, but if this is
some intentional pattern, please explain it to me.
In a few places where nullness is statically known but non-obvious,
add assert statements to point out what's going on to help future
developers.
Upgrade future such warnings to errors to keep us moving in a cleaner
direction.
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