- Default constructor scopes won't need the scope data for deciding the scope
allocation of variables inside them. Also, PreParser doesn't construct them. So
they should be just skipped when applying the scope data.

- PreParser needs to declare the class name + have a proper end position for
the class scope.

- This makes all mjsunit tests pass with --experimental-preparser-scope-analysis.

- Also added several DCHECKs which were useful for debugging.

BUG=v8:5516

Change-Id: I5b3e6c60ed79efe25f33576a3547d707c700c6dd
Reviewed-on: https://chromium-review.googlesource.com/503208
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45303}