Create a main alias for master channel. (#115388)

* Create a main alias for master channel.

To slowly migrate away from master branch in the flutter repository we
created a main branch that is mirroring master branch. This PR is also
adding a channel alias that will allow to use master/main interchangeably.

Bug: flutter/flutter#95041

* Fix channel tests.

* Remove additional space.

packages/flutter_tools/lib/src/persistent_tool_state.dart

@@ -83,6 +83,7 @@ class _DefaultPersistentToolState implements PersistentToolState {
   static const String _kRedisplayWelcomeMessage = 'redisplay-welcome-message';
   static const Map<Channel, String> _lastActiveVersionKeys = <Channel,String>{
     Channel.master: 'last-active-master-version',
+    Channel.main: 'last-active-main-version',
     Channel.beta: 'last-active-beta-version',
     Channel.stable: 'last-active-stable-version',
   };

packages/flutter_tools/lib/src/version.dart

@@ -32,15 +32,16 @@ const Map<String, String> kObsoleteBranches = <String, String>{
 
 /// The names of each channel/branch in order of increasing stability.
 enum Channel {
-  // TODO(fujino): update to main https://github.com/flutter/flutter/issues/95041
   master,
+  main,
   beta,
   stable,
 }
 
 // Beware: Keep order in accordance with stability
 const Set<String> kOfficialChannels = <String>{
   globals.kDefaultFrameworkChannel,
+  'main',
   'beta',
   'stable',
 };

packages/flutter_tools/test/general.shard/channel_test.dart

@@ -67,6 +67,7 @@ void main() {
           command: <String>['git', 'branch', '-r'],
           stdout: 'origin/beta\n'
               'origin/master\n'
+              'origin/main\n'
               'origin/stable\n',
         ),
       );
@@ -90,6 +91,7 @@ void main() {
           stdout: 'origin/beta\n'
               'origin/master\n'
               'origin/dependabot/bundler\n'
+              'origin/main\n'
               'origin/v1.4.5-hotfixes\n'
               'origin/stable\n',
         ),