Fix publish-check output (#3953)

script/tool/lib/src/publish_check_command.dart

@@ -175,7 +175,10 @@ class PublishCheckCommand extends PluginCommand {
       (List<int> event) {
         final String output = String.fromCharCodes(event);
         if (output.isNotEmpty) {
-          _printImportantStatusMessage(output, isError: true);
+          // The final result is always printed on stderr, whether success or
+          // failure.
+          final bool isError = !output.contains('has 0 warnings');
+          _printImportantStatusMessage(output, isError: isError);
           outputBuffer.write(output);
         }
       },

script/tool/test/publish_check_command_test.dart

@@ -132,6 +132,26 @@ void main() {
       expect(runner.run(<String>['publish-check']), throwsA(isA<ToolExit>()));
     });
 
+    test('Success message on stderr is not printed as an error', () async {
+      createFakePlugin('d');
+
+      const String publishOutput = 'Package has 0 warnings.';
+
+      final MockProcess process = MockProcess();
+      process.stderrController.add(publishOutput.codeUnits);
+      process.stdoutController.close(); // ignore: unawaited_futures
+      process.stderrController.close(); // ignore: unawaited_futures
+
+      process.exitCodeCompleter.complete(0);
+
+      processRunner.processesToReturn.add(process);
+
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['publish-check']);
+
+      expect(output, isNot(contains(contains('ERROR:'))));
+    });
+
     test(
         '--machine: Log JSON with status:no-publish and correct human message, if there are no packages need to be published. ',
         () async {