[dds/dap] Prevent duplicate values showing in variables views when a class has both a field and a getter with the same name

When a class contains both a field and getter with the same name, we'd include both in the `variables` response. In most cases these are the same value (although it's not guaranteed).

I've chosen to just hide the field in this case and always show the result from evaluating the getter (since I think that's what the user would expect, even in the case where they happen to have different values). Another option could be to show both (but change the name so that fields/getters are shown differently), however in that would change the display (for example adding `get ` in front of all getters) we should probably only do that if it's clear there is demand for it.

Fixes https://github.com/Dart-Code/Dart-Code/issues/5128

Change-Id: I9e23d22a844ee22c38988456b1f275422c5c9e04
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/370640
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: Helin Shiah <helinx@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>