Detail number of available devices in doctor summary (#14781)

packages/flutter_tools/lib/src/doctor.dart

diff --git a/packages/flutter_tools/lib/src/doctor.dart b/packages/flutter_tools/lib/src/doctor.dart
index a357560..d4f8a15 100644
--- a/packages/flutter_tools/lib/src/doctor.dart
+++ b/packages/flutter_tools/lib/src/doctor.dart
@@ -542,7 +542,12 @@
       messages = await Device.descriptions(devices)
           .map((String msg) => new ValidationMessage(msg)).toList();
     }
-    return new ValidationResult(devices.isEmpty ? ValidationType.partial : ValidationType.installed, messages);
+
+    if (devices.isEmpty) {
+      return new ValidationResult(ValidationType.partial, messages);
+    } else {
+      return new ValidationResult(ValidationType.installed, messages, statusInfo: '${devices.length} available');
+    }
   }
 }