[tools] add Stdio to AppContext created by daemon AppInstance (#15268)

packages/flutter_tools/lib/src/commands/daemon.dart

diff --git a/packages/flutter_tools/lib/src/commands/daemon.dart b/packages/flutter_tools/lib/src/commands/daemon.dart
index e374eda..46c1373 100644
--- a/packages/flutter_tools/lib/src/commands/daemon.dart
+++ b/packages/flutter_tools/lib/src/commands/daemon.dart
@@ -812,6 +812,7 @@
 
     final AppContext appContext = new AppContext();
     appContext.setVariable(Logger, _logger);
+    appContext.setVariable(Stdio, const Stdio());
     return appContext.runInZone(method);
   }
 }