Help verbose show hidden options (#5979)

* show hidden flags when showing verbose help
* flutter -v show verbose help

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 7351349..ace05fd 100644
--- a/packages/flutter_tools/lib/src/commands/build.dart
+++ b/packages/flutter_tools/lib/src/commands/build.dart
@@ -17,12 +17,12 @@
 import 'build_ios.dart';
 
 class BuildCommand extends FlutterCommand {
-  BuildCommand() {
+  BuildCommand({bool verboseHelp: false}) {
     addSubcommand(new BuildApkCommand());
     addSubcommand(new BuildAotCommand());
     addSubcommand(new BuildCleanCommand());
     addSubcommand(new BuildIOSCommand());
-    addSubcommand(new BuildFlxCommand());
+    addSubcommand(new BuildFlxCommand(verboseHelp: verboseHelp));
   }
 
   @override