Make DeviceDomain echo a msg if doctor says it can't list anything (#9749)

This message will be picked up by IntelliJ and shown to the user in a toast.
diff --git a/packages/flutter_tools/lib/src/device.dart b/packages/flutter_tools/lib/src/device.dart
index b356690..6898806 100644
--- a/packages/flutter_tools/lib/src/device.dart
+++ b/packages/flutter_tools/lib/src/device.dart
@@ -92,6 +92,11 @@
 /// An abstract class to discover and enumerate a specific type of devices.
 abstract class DeviceDiscovery {
   bool get supportsPlatform;
+
+  /// Whether this device discovery is capable of listing any devices given the
+  /// current environment configuration.
+  bool get canListAnything;
+
   List<Device> get devices;
 }