mock SDK: Make whitespace consistent

Not the most impactful CL, but. When we go to generating mocks, the
whitespace will become consistent. We can either have a blank line
between all declarations, or we can bunch them together. Or we can
have bespoke rules about one-line declarations or constructors or
fields, etc.

I don't have an opinion, but I'm leading with this CL to put a blank
line between all declarations.

Change-Id: I718a48977e39f3f8560e1dc9c874c856f56622dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/463164
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: srawlins