Include git output into error message from channel command (#28658)

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

@@ -35,7 +35,10 @@ class ChannelCommand extends FlutterCommand {
   Future<FlutterCommandResult> runCommand() async {
     switch (argResults.rest.length) {
       case 0:
-        await _listChannels(showAll: argResults['all']);
+        await _listChannels(
+          showAll: argResults['all'],
+          verbose: globalResults['verbose']
+        );
         return null;
       case 1:
         await _switchChannel(argResults.rest[0]);
@@ -45,11 +48,12 @@ class ChannelCommand extends FlutterCommand {
     }
   }
 
-  Future<void> _listChannels({ bool showAll }) async {
+  Future<void> _listChannels({ bool showAll, bool verbose }) async {
     // Beware: currentBranch could contain PII. See getBranchName().
     final String currentChannel = FlutterVersion.instance.channel;
     final String currentBranch = FlutterVersion.instance.getBranchName();
     final Set<String> seenChannels = Set<String>();
+    final List<String> rawOutput = <String>[];
 
     showAll = showAll || currentChannel != currentBranch;
 
@@ -58,6 +62,8 @@ class ChannelCommand extends FlutterCommand {
       <String>['git', 'branch', '-r'],
       workingDirectory: Cache.flutterRoot,
       mapFunction: (String line) {
+        if (verbose)
+          rawOutput.add(line);
         final List<String> split = line.split('/');
         if (split.length < 2)
           return null;
@@ -74,8 +80,10 @@ class ChannelCommand extends FlutterCommand {
         return null;
       },
     );
-    if (result != 0)
-      throwToolExit('List channels failed: $result', exitCode: result);
+    if (result != 0) {
+      final String details = verbose ? '\n${rawOutput.join('\n')}' : '';
+      throwToolExit('List channels failed: $result$details', exitCode: result);
+    }
   }
 
   Future<void> _switchChannel(String branchName) {

packages/flutter_tools/test/channel_test.dart

@@ -41,15 +41,23 @@ void main() {
       Cache.disableLocking();
     });
 
-    testUsingContext('list', () async {
+    Future<void> simpleChannelTest(List<String> args) async {
       final ChannelCommand command = ChannelCommand();
       final CommandRunner<void> runner = createTestCommandRunner(command);
-      await runner.run(<String>['channel']);
+      await runner.run(args);
       expect(testLogger.errorText, hasLength(0));
       // The bots may return an empty list of channels (network hiccup?)
       // and when run locally the list of branches might be different
       // so we check for the header text rather than any specific channel name.
       expect(testLogger.statusText, contains('Flutter channels:'));
+    }
+
+    testUsingContext('list', () async {
+      await simpleChannelTest(<String>['channel']);
+    });
+
+    testUsingContext('verbose list', () async {
+      await simpleChannelTest(<String>['channel', '-v']);
     });
 
     testUsingContext('removes duplicates', () async {