Breakpoints in web apps don't jump to any source #2784

When running a (non-Flutter) web app like this (the delay is to avoid dart-lang/webdev#830):

void main() {
  querySelector('#output').text = 'Your Dart app is running.';
  // BREAKPOINT1^

  Future.delayed(const Duration(seconds: 5)).then((_) {
    // Breakpoint on the line below
    querySelector('#output').text = 'Delayed function has run!';
  });
}

When the breakpoint is hit, we don't jump to any code and the stack looks like this:

VM service shows that we can get sources for at least one of these frames, even though we can't map it back to a local file (since it's an org-dartlang URI).