DAS: Tidy the replay script

Previously, the Driver class here was instantiated with no parameters,
as an empty object, and then in the `_processCommandLine` method
(called once by `run`), we set all the fields. These fields in
practice are immutable and do not represent any "state" of running the
Driver. This is not taking advantage of any OO practices or our best
practices of using final fields.

This CL moves the argument-parsing logic to a static method, `Driver.fromArgs`. This can't be a factory constructor because some
args do result in no Driver being returned (`--help`, not enough args,
and an exception being thrown). Then the values of the flags can be
final fields, set in the private constructor. Then, `run()` does not
set fields or read arguments; it just runs :D

Additionally, the fields can be made private.

Additionally, the command-line argument name constants should be
lowerCamelCase.

Additionally, `_createArgParser` does not need to be an instance
method; it makes more sense as a static getter.

Change-Id: If3b0363859f35b87dfdbcedf43b472bf5689b200
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/454402
Reviewed-by: Keerti Parthasarathy <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins