[flutter_tools] Improve toolexit error messages for `flutter upgrade` (#86351)

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

@@ -17,6 +17,9 @@ import '../globals_null_migrated.dart' as globals;
 import '../runner/flutter_command.dart';
 import '../version.dart';
 
+// The official docs to install Flutter.
+const String _flutterInstallDocs = 'https://flutter.dev/docs/get-started/install';
+
 class UpgradeCommand extends FlutterCommand {
   UpgradeCommand({
     @required bool verboseHelp,
@@ -224,7 +227,7 @@ class UpgradeCommandRunner {
 
   /// Returns the remote HEAD flutter version.
   ///
-  /// Exits tool if there is no upstream.
+  /// Exits tool if HEAD isn't pointing to a branch, or there is no upstream.
   Future<FlutterVersion> fetchLatestVersion() async {
     String revision;
     try {
@@ -245,16 +248,17 @@ class UpgradeCommandRunner {
       final String errorString = e.toString();
       if (errorString.contains('fatal: HEAD does not point to a branch')) {
         throwToolExit(
-          'You are not currently on a release branch. Use git to '
-          "check out an official branch ('stable', 'beta', 'dev', or 'master') "
-          'and retry, for example:\n'
-          '  git checkout stable'
+          'Unable to upgrade Flutter: Your Flutter checkout is currently not '
+          'on a release branch.\n'
+          'Use "flutter channel" to switch to an official channel, and retry. '
+          'Alternatively, re-install Flutter by going to $_flutterInstallDocs.'
         );
       } else if (errorString.contains('fatal: no upstream configured for branch')) {
         throwToolExit(
-          'Unable to upgrade Flutter: no origin repository configured. '
-          "Run 'git remote add origin "
-          "https://github.com/flutter/flutter' in $workingDirectory");
+          'Unable to upgrade Flutter: The current Flutter branch/channel is '
+          'not tracking any remote repository.\n'
+          'Re-install Flutter by going to $_flutterInstallDocs.'
+        );
       } else {
         throwToolExit(errorString);
       }

packages/flutter_tools/test/commands.shard/permeable/upgrade_test.dart

@@ -199,8 +199,12 @@ void main() {
       ]);
 
       await expectLater(
-            () async => realCommandRunner.fetchLatestVersion(),
-        throwsToolExit(message: 'You are not currently on a release branch.'),
+        () async => realCommandRunner.fetchLatestVersion(),
+        throwsToolExit(message: 'Unable to upgrade Flutter: Your Flutter checkout '
+          'is currently not on a release branch.\n'
+          'Use "flutter channel" to switch to an official channel, and retry. '
+          'Alternatively, re-install Flutter by going to https://flutter.dev/docs/get-started/install.'
+        ),
       );
       expect(processManager, hasNoRemainingExpectations);
     }, overrides: <Type, Generator>{
@@ -208,7 +212,7 @@ void main() {
       Platform: () => fakePlatform,
     });
 
-    testUsingContext('fetchRemoteRevision throws toolExit if no upstream configured', () async {
+    testUsingContext('fetchLatestVersion throws toolExit if no upstream configured', () async {
       processManager.addCommands(const <FakeCommand>[
         FakeCommand(command: <String>[
           'git', 'fetch', '--tags'
@@ -224,9 +228,10 @@ void main() {
       ]);
 
       await expectLater(
-            () async => realCommandRunner.fetchLatestVersion(),
-        throwsToolExit(
-          message: 'Unable to upgrade Flutter: no origin repository configured.',
+        () async => realCommandRunner.fetchLatestVersion(),
+        throwsToolExit(message: 'Unable to upgrade Flutter: The current Flutter '
+          'branch/channel is not tracking any remote repository.\n'
+          'Re-install Flutter by going to https://flutter.dev/docs/get-started/install.'
         ),
       );
       expect(processManager, hasNoRemainingExpectations);