DAS: Rearrange code to promote variables better

The motivation here was to remove the local duplicate variables like
`final_foo` or `foo_final`. These variables only existed because the
variables they duplicate lose their promoted types inside the
`builder.addDartFileEdit` closure. They lose their promoted type
because they are multiply assigned in loops or not always promoted to
be non-null.

Often the fix is to replace a `if (x == null) return` with
`} else { return; }`. I think this code more directly represents the
flow of code, rather than relying on the nullity of a variable, like
"is this variable still null? Oh then we must not have entered any
of the situations above, so we should return."

Change-Id: I9dd6686b7f4d9c6caf59c171179b129a5a957ba8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418160
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>