Observatory: Don't give a stacktrace on Sentinel

E.g. on this:
```
import "dart:developer";

class C {
  foo2() {
    var temp = 1;
    [1, 2, 3].forEach((outer_ea){
      [1, 2, 3].forEach((inner_ea){
         debugger();
         throw "Two!";
      });
    });
  }
}

main() {
  new C().foo2();
}
```

Saying `p this` in the debugger when it stops now says `<optimized out>`
instead of something like
```
Internal error: TypeError: Instance of 'qp': type 'qp' is not a subtype of type 'b6'

TypeError: Instance of 'qp': type 'qp' is not a subtype of type 'b6'
    at Object.m (http://127.0.0.1:54975/main.dart.js:1630:3)
    at Object.os (http://127.0.0.1:54975/main.dart.js:1830:26)
    at Object.b (http://127.0.0.1:54975/main.dart.js:1835:3)
    at http://127.0.0.1:54975/main.dart.js:26687:14
    at a1I.a (http://127.0.0.1:54975/main.dart.js:3202:71)
    at a1I.$2 (http://127.0.0.1:54975/main.dart.js:3547:23)
    at TB.$0 (http://127.0.0.1:54975/main.dart.js:39207:33)
    at Tz.lp (http://127.0.0.1:54975/main.dart.js:39181:9)
    at TC.$2 (http://127.0.0.1:54975/main.dart.js:39205:15)
    at TB.$0 (http://127.0.0.1:54975/main.dart.js:39207:33)
```
Change-Id: I7e001816742ff84dad5859f83b08da86d1043ccf
Reviewed-on: https://dart-review.googlesource.com/68660
Reviewed-by: Kevin Millikin <kmillikin@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
1 file changed
tree: fd8b054dc10249094f07d728717a71c9068895da
  1. .clang-format
  2. .gitattributes
  3. .gitconfig
  4. .github/
  5. .gitignore
  6. .gn
  7. .mailmap
  8. .packages
  9. AUTHORS
  10. BUILD.gn
  11. CHANGELOG.md
  12. DEPS
  13. LICENSE
  14. PATENTS
  15. PRESUBMIT.py
  16. README.dart-sdk
  17. README.md
  18. WATCHLISTS
  19. build/
  20. client/
  21. codereview.settings
  22. dartdoc_options.yaml
  23. docs/
  24. pkg/
  25. runtime/
  26. samples-dev/
  27. samples/
  28. sdk/
  29. tests/
  30. third_party/
  31. tools/
  32. utils/
README.md

Dart

Dart is an open-source, scalable programming language, with robust libraries and runtimes, for building web, server, and mobile apps.

Using Dart

Visit the dartlang.org to learn more about the language, tools, getting started, and more.

Browse pub.dartlang.org for more packages and libraries contributed by the community and the Dart team.

Building Dart

If you want to build Dart yourself, here is a guide to getting the source, preparing your machine to build the SDK, and building.

There are more documents on our wiki.

Contributing to Dart

The easiest way to contribute to Dart is to file issues.

You can also contribute patches, as described in Contributing.

License & patents

See LICENSE and PATENTS.