The debugger screen should still be available when no compilation service is present

When attempting to debug a test in the Flutter engine repo, @dnfield was unable to find the Debugger screen when attaching to the test with DevTools.

**Reproduction steps:**

1: Make a hello_world.dart: void main() { print('hello world'); }
2: dart compile kernel hello_world.dart
3: /path/to/flutter_sdk/bin/cache/artifacts/engine/darwin-x64/flutter_tester --enable-observatory --start-paused hello_world.dill
4: Navigate to printed URL, get error message
5: run dart devtools and input that URL
6: No debugger tab.

@jason-simmons pointed out that this tab is being hidden because the check for whether or not the target application is in debug mode fails due to the absence of a compilation service:

> DevTools is not showing the debugger tab because its check for whether the app is in debug mode is failing.
>
> The check is trying to evaluate an expression in the target VM. But that fails with a No compilation service available; cannot evaluate from source. error.

Typically the Flutter tool registers the compilation service, so it's not available when running tests with `flutter_tester` directly.

The lack of a compilation service shouldn't result in the debugger screen being hidden but should instead only result in functionality requiring expression evaluation being disabled.

FYI @elliette @kenzieschmoll