Revert "observatory: Ignore new static Warning"

This reverts commit d3124b8.

Reason for revert: unknown code for now.

Original change's description:
> observatory: Ignore new static Warning
>
> There are two spots where a nullable value is passed to a non-nullable
> Future.value call, or a non-nullable Completer.complete call. In the
> interest of not making any functional change, I'm just ignoring this
> new code.
>
> ```
> warning - lib/src/service/object.dart:870:29 - 'Future<Isolate>.value' shouldn't be called with an argument of nullable type 'Isolate?'. Try passing a non-null argument. - nullable_argument_to_non_null_type
> warning - lib/src/service/object.dart:1559:29 - 'Future<Class>.value' shouldn't be called with an argument of nullable type 'Class?'. Try passing a non-null argument. - nullable_argument_to_non_null_type
> ```
>
> Related to #53253
>
> TEST=N/A
>
> Change-Id: I091990975a1f2b927e007564b6ca0658c6a34c20
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324762
> Reviewed-by: Ben Konyi <bkonyi@google.com>
> Auto-Submit: Samuel Rawlins <srawlins@google.com>
> Commit-Queue: Samuel Rawlins <srawlins@google.com>

Change-Id: Id5d033ca8601ea2f18aa9d2c8f8aeee8a2ceaf0d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/326725
Commit-Queue: Alexander Thomas <athom@google.com>
Auto-Submit: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

runtime/observatory/analysis_options.yaml

@@ -3,7 +3,6 @@ analyzer:
     dead_code: info
     getter_not_subtype_setter_types: info
     missing_enum_constant_in_switch: info
-    nullable_argument_to_non_null_type: info
     unused_element: info
     unused_field: info
     unused_import: info