[ddc] Refactor hot reload suite

Break body of main method into instance method helpers on a suite
runner class. In upcoming refactor changes this will become an
abstract base class that runtime specific subclasses will inherit
from or provide their own custom implementation for compiling,
accepting/rejecting edits, and running the tests.

Change-Id: I81541c48e97f22f034a4529a96d2af53546362ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393080
Reviewed-by: Mark Zhou <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan