VM `unlinked_ast_to_text_test` failure in CL

[lrhn]

I'm working on https://dart-review.googlesource.com/c/sdk/+/425862.

The current CL makes pkg/vm/test/unlinked_ast_to_text_test fail.
The error starts with:

Unhandled exception:
Reference to dart:async::ZoneDelegate is not bound to an AST node. An extension type declaration was expected
#0      Reference.asExtensionTypeDeclaration (package:kernel/canonical_name.dart:559:7)
#1      ExtensionType.extensionTypeDeclaration (package:kernel/src/ast/types.dart:1439:41)
#2      ExtensionType.nullability (package:kernel/src/ast/types.dart:1468:16)
#3      Printer.visitExtensionType (package:kernel/text/ast_to_text.dart:2639:27)

I don't know why there is no AST node for the ZoneDelegate extension type, it's in the source.

Marking the test as failing.

[a-siva]

//cc @vegorov do you think the change in flight impacts they way your async stack walker implementation was done.