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

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));
   }
 

diff --git a/packages/flutter_tools/lib/src/commands/build_aot.dart b/packages/flutter_tools/lib/src/commands/build_aot.dart
index 367fe0d..4d766bf 100644
--- a/packages/flutter_tools/lib/src/commands/build_aot.dart
+++ b/packages/flutter_tools/lib/src/commands/build_aot.dart

@@ -27,7 +27,7 @@
 ];
 
 class BuildAotCommand extends BuildSubCommand {
-  BuildAotCommand() {
+  BuildAotCommand({bool verboseHelp: false}) {
     usesTargetOption();
     addBuildModeFlags();
     usesPubOption();
@@ -39,7 +39,7 @@
       )
       ..addFlag('interpreter')
       ..addFlag('quiet', defaultsTo: false)
-      ..addFlag('preview-dart-2', negatable: false)
+      ..addFlag('preview-dart-2', negatable: false, hide: !verboseHelp)
       ..addOption(FlutterOptions.kExtraFrontEndOptions,
         allowMultiple: true,
         splitCommas: true,

diff --git a/packages/flutter_tools/lib/src/commands/build_apk.dart b/packages/flutter_tools/lib/src/commands/build_apk.dart
index ff53ab7..0c1811d 100644
--- a/packages/flutter_tools/lib/src/commands/build_apk.dart
+++ b/packages/flutter_tools/lib/src/commands/build_apk.dart

@@ -8,14 +8,14 @@
 import 'build.dart';
 
 class BuildApkCommand extends BuildSubCommand {
-  BuildApkCommand() {
+  BuildApkCommand({bool verboseHelp: false}) {
     usesTargetOption();
     addBuildModeFlags();
     usesFlavorOption();
     usesPubOption();
 
     argParser
-      ..addFlag('preview-dart-2', negatable: false)
+      ..addFlag('preview-dart-2', negatable: false,  hide: !verboseHelp)
       ..addFlag('prefer-shared-library', negatable: false,
           help: 'Whether to prefer compiling to a *.so file (android only).');
   }

diff --git a/packages/flutter_tools/lib/src/commands/build_flx.dart b/packages/flutter_tools/lib/src/commands/build_flx.dart
index 9188cd5..be66079 100644
--- a/packages/flutter_tools/lib/src/commands/build_flx.dart
+++ b/packages/flutter_tools/lib/src/commands/build_flx.dart

@@ -20,7 +20,7 @@
     argParser.addOption('output-file', abbr: 'o', defaultsTo: defaultFlxOutputPath);
     argParser.addOption('snapshot', defaultsTo: defaultSnapshotPath);
     argParser.addOption('depfile', defaultsTo: defaultDepfilePath);
-    argParser.addFlag('preview-dart-2', negatable: false);
+    argParser.addFlag('preview-dart-2', negatable: false, hide: !verboseHelp);
     argParser.addOption('working-dir', defaultsTo: getAssetBuildDirectory());
     argParser.addFlag('report-licensed-packages', help: 'Whether to report the names of all the packages that are included in the application\'s LICENSE file.', defaultsTo: false);
     usesPubOption();

diff --git a/packages/flutter_tools/lib/src/commands/build_ios.dart b/packages/flutter_tools/lib/src/commands/build_ios.dart
index add0a83..17fab26 100644
--- a/packages/flutter_tools/lib/src/commands/build_ios.dart
+++ b/packages/flutter_tools/lib/src/commands/build_ios.dart

@@ -13,7 +13,7 @@
 import 'build.dart';
 
 class BuildIOSCommand extends BuildSubCommand {
-  BuildIOSCommand() {
+  BuildIOSCommand({bool verboseHelp: false}) {
     usesTargetOption();
     usesFlavorOption();
     usesPubOption();
@@ -29,7 +29,8 @@
     argParser.addFlag('simulator', help: 'Build for the iOS simulator instead of the device.');
     argParser.addFlag('codesign', negatable: true, defaultsTo: true,
         help: 'Codesign the application bundle (only available on device builds).');
-    argParser.addFlag('preview-dart-2', negatable: false);
+    argParser.addFlag('preview-dart-2', negatable: false,
+        hide: !verboseHelp);
   }
 
   @override