Revert "[ VM / Service ] Add abstract flag to Function"

This reverts commit 921476edf3.

Reason for revert: breaks google3 (b/236964692).

Original change's description:
> [ VM / Service ] Add abstract flag to Function
>
> Bug: https://github.com/dart-lang/coverage/issues/398
> Change-Id: I1f6e16483e28fb7a77aebae079fd62abe9d16099
> TEST=Added to get_object_rpc_test.dart
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249421
> Reviewed-by: Ben Konyi <bkonyi@google.com>
> Commit-Queue: Liam Appelbe <liama@google.com>

TBR=bkonyi@google.com,liama@google.com

Change-Id: I1733080dadd6f283766dddbdbe1390d821f91f64
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: https://github.com/dart-lang/coverage/issues/398
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249680
Commit-Queue: Emmanuel Pellereau <emmanuelp@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Emmanuel Pellereau <emmanuelp@google.com>

pkg/vm_service/java/version.properties

@@ -1 +1 @@
-version=3.59
+version=3.58