Hide preview-dart-2 flag. Show only in verbose (#13270)

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

diff --git a/packages/flutter_tools/lib/src/commands/build.dart b/packages/flutter_tools/lib/src/commands/build.dart
index 61ebf86..49f2195 100644
--- a/packages/flutter_tools/lib/src/commands/build.dart
+++ b/packages/flutter_tools/lib/src/commands/build.dart
@@ -17,9 +17,9 @@
 
 class BuildCommand extends FlutterCommand {
   BuildCommand({bool verboseHelp: false}) {
-    addSubcommand(new BuildApkCommand());
-    addSubcommand(new BuildAotCommand());
-    addSubcommand(new BuildIOSCommand());
+    addSubcommand(new BuildApkCommand(verboseHelp: verboseHelp));
+    addSubcommand(new BuildAotCommand(verboseHelp: verboseHelp));
+    addSubcommand(new BuildIOSCommand(verboseHelp: verboseHelp));
     addSubcommand(new BuildFlxCommand(verboseHelp: verboseHelp));
   }