vm_service: Assign fields in field initializers

This is achieved with just a bit of delicacy around commas, braces,
semicolons, and possible super-initializers.

There is exactly one case of a statement that needs to remain in the
constructor body: `_parseTokenPosTable()`.

Additionally I add one helper, `_createServiceObjectListOrNull`, which takes care of some casting and nullability quirks. This also includes one small perf improvement: when `createServiceObject` returns null,
we immediately return an empty list, without wrapping it in `List.from`.


vm_service: helper for list fields
Change-Id: Ic5e36bbfa451bea06edf0d9e684705d392f1ad39
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436540
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>

Author: srawlins