Improve 'flutter downgrade' error message (flutter#154434)

`flutter downgrade` fails if you haven't used `flutter upgrade`:

```
$ flutter downgrade
There is no previously recorded version for channel "stable".
```

It's not clear what actions a user should take from this error message. Here's the new error message:

```
$ flutter downgrade
It looks like you haven't run "flutter upgrade" on channel "stable".

"flutter downgrade" undoes the last "flutter upgrade".

To switch to a specific Flutter version, see: https://flutter.dev/to/switch-flutter-version
```

Depends on flutter/website#11098

Author: loic-sharma

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

@@ -110,9 +110,18 @@ class DowngradeCommand extends FlutterCommand {
     final String? lastFlutterVersion = persistentToolState.lastActiveVersion(channel);
     final String? currentFlutterVersion = _flutterVersion?.frameworkRevision;
     if (lastFlutterVersion == null || currentFlutterVersion == lastFlutterVersion) {
-      final String trailing = await _createErrorMessage(workingDirectory, channel);
+      final String trailing = await _createErrorMessage(
+        workingDirectory,
+        channel,
+      );
       throwToolExit(
-        'There is no previously recorded version for channel "$currentChannel".\n'
+        "It looks like you haven't run "
+        '"flutter upgrade" on channel "$currentChannel".\n'
+        '\n'
+        '"flutter downgrade" undoes the last "flutter upgrade".\n'
+        '\n'
+        'To switch to a specific Flutter version, see: '
+        'https://flutter.dev/to/switch-flutter-version'
         '$trailing'
       );
     }
@@ -181,7 +190,10 @@ class DowngradeCommand extends FlutterCommand {
   }
 
   // Formats an error message that lists the currently stored versions.
-  Future<String> _createErrorMessage(String workingDirectory, Channel currentChannel) async {
+  Future<String> _createErrorMessage(
+    String workingDirectory,
+    Channel currentChannel,
+  ) async {
     final StringBuffer buffer = StringBuffer();
     for (final Channel channel in Channel.values) {
       if (channel == currentChannel) {
@@ -191,11 +203,19 @@ class DowngradeCommand extends FlutterCommand {
       if (sha == null) {
         continue;
       }
-      final RunResult parseResult = await _processUtils!.run(<String>[
-        'git', 'describe', '--tags', sha,
-      ], workingDirectory: workingDirectory);
+      final RunResult parseResult = await _processUtils!.run(
+        <String>['git', 'describe', '--tags', sha],
+        workingDirectory: workingDirectory,
+      );
       if (parseResult.exitCode == 0) {
-        buffer.writeln('Channel "${getNameForChannel(channel)}" was previously on: ${parseResult.stdout}.');
+        if (buffer.isEmpty) {
+          buffer.writeln();
+        }
+        buffer.writeln();
+        buffer.writeln(
+          'Channel "${getNameForChannel(channel)}" was previously on: '
+          '${parseResult.stdout}.'
+        );
       }
     }
     return buffer.toString();

packages/flutter_tools/test/commands.shard/hermetic/downgrade_test.dart

@@ -77,10 +77,16 @@ void main() {
       logger: bufferLogger,
     );
 
-    expect(createTestCommandRunner(command).run(const <String>['downgrade']),
-      throwsToolExit(message:
-        'There is no previously recorded version for channel "beta".\n'
-        'Channel "master" was previously on: v1.2.3.'
+    expect(
+      createTestCommandRunner(command).run(const <String>['downgrade']),
+      throwsToolExit(message: '''
+It looks like you haven't run "flutter upgrade" on channel "beta".
+
+"flutter downgrade" undoes the last "flutter upgrade".
+
+To switch to a specific Flutter version, see: https://flutter.dev/to/switch-flutter-version
+
+Channel "master" was previously on: v1.2.3.''',
       ),
     );
   });