[ dartfuzz ] Second half of DartFuzz refactor

- Indentation and whitespace should be more consistent
- Abstracted out common emit patterns (e.g., variable declaration and
initialization)

Should be structurally and logically equivalent to prior implementation
other than whitespace.

Change-Id: I47e450e97b4697efc0a2f346160efeee2ab01a44
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121646
Reviewed-by: Aart Bik <ajcbik@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

Author: bkonyi