[gardening] Make new test work also in obfuscation modified

The new `lib/isolate/handle_throwing_to_string_error_test` was relying on
symbol names not being obfuscated and fails on
`vm-aot-obfuscate-linux-release-x64`.

=> Make it more robust by using `Symbol`s instead of `Strings`.

Change-Id: Ib1cd793b100b1c0b31f0a909f4ec8db442590540
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413660
Reviewed-by: Daco Harkes <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

tests/lib/isolate/handle_throwing_to_string_error_test.dart

@@ -32,8 +32,17 @@ void main() async {
   Isolate.spawn((_) => isolateMain(), null, onError: errorPort.sendPort);
   var errorAndStack = await errorPort.first as List<Object?>;
   Expect.listEquals([
-    "Instance of 'ThrowingToStringError'",
-    "Instance of 'ThrowingToStringStackTrace'",
+    "Instance of '${(#ThrowingToStringError).name}'",
+    "Instance of '${(#ThrowingToStringStackTrace).name}'",
   ], errorAndStack);
   asyncEnd();
 }
+
+final symbolToStringPattern = RegExp(r'^Symbol[(]"(?<name>.+)"[)]$');
+
+extension on Symbol {
+  String get name {
+    final match = symbolToStringPattern.firstMatch(toString())!;
+    return match.namedGroup('name')!;
+  }
+}