Can't attach to debugger on Dart VM running frontend_server

I'm not sure the exact scenario required to trigger this, but attempting to connect to the VM Service of a frontend_server snapshot was sufficient to reproduce it. Debugging from the Observatory works.

To get the Observatory to print out:

```diff
diff --git a/packages/flutter_tools/lib/src/compile.dart b/packages/flutter_tools/lib/src/compile.dart
index 76ba6793b8..bc681ee8c9 100644
--- a/packages/flutter_tools/lib/src/compile.dart
+++ b/packages/flutter_tools/lib/src/compile.dart
@@ -696,11 +696,13 @@ class DefaultResidentCompiler implements ResidentCompiler {
);
final List command = [
_artifacts.getHostArtifact(HostArtifact.engineDartBinary).path,
+ '--observe',
'--disable-dart-dev',
frontendServer,
'--sdk-root',
sdkRoot,
'--incremental',
+ '--verbose',
if (testCompilation)
'--no-print-incremental-dependencies',
'--target=$targetModel',
```

To trigger the crash I'm trying to debug (requires running a flutter web app)

```diff
diff --git a/packages/flutter_tools/lib/src/resident_runner.dart b/packages/flutter_tools/lib/src/resident_runner.dart
index 0342a56138..786db092cd 100644
--- a/packages/flutter_tools/lib/src/resident_runner.dart
+++ b/packages/flutter_tools/lib/src/resident_runner.dart
@@ -116,6 +116,7 @@ class FlutterDevice {
} else {
assert(false);
}
+ extraFrontEndOptions.add('--enable-experiment=alternative-invalidation-strategy');

generator = ResidentCompiler(
globals.artifacts.getHostArtifact(HostArtifact.flutterWebSdk).path,
```

Note that it says it is paused in the debugger, but there is no debugger pane and hitting step doesn't seem to do anything.