fix noop toString() diagnostics (#108836)

packages/flutter_tools/lib/src/migrate/migrate_manifest.dart

@@ -171,7 +171,7 @@ class MigrateManifest {
       deletedFileManifestContents.write('  - $localPath\n');
     }
 
-    final String migrateManifestContents = 'merged_files:\n${mergedFileManifestContents.toString()}conflict_files:\n${conflictFilesManifestContents.toString()}added_files:\n${newFileManifestContents.toString()}deleted_files:\n${deletedFileManifestContents.toString()}';
+    final String migrateManifestContents = 'merged_files:\n${mergedFileManifestContents}conflict_files:\n${conflictFilesManifestContents}added_files:\n${newFileManifestContents}deleted_files:\n$deletedFileManifestContents';
     final File migrateManifest = getManifestFileFromDirectory(migrateRootDir);
     migrateManifest.createSync(recursive: true);
     migrateManifest.writeAsStringSync(migrateManifestContents, flush: true);

packages/flutter_tools/test/integration.shard/build_ios_config_only_test.dart

@@ -40,7 +40,7 @@ void main() {
     printOnFailure('Output of flutter build ios:');
     final String firstRunStdout = firstRunResult.stdout.toString();
     printOnFailure('First run stdout: $firstRunStdout');
-    printOnFailure('First run stderr: ${firstRunResult.stderr.toString()}');
+    printOnFailure('First run stderr: ${firstRunResult.stderr}');
 
     expect(firstRunResult.exitCode, 0);
     expect(firstRunStdout, contains('Running pod install'));
@@ -72,7 +72,7 @@ void main() {
     final ProcessResult secondRunResult = await processManager.run(buildCommand, workingDirectory: workingDirectory);
     final String secondRunStdout = secondRunResult.stdout.toString();
     printOnFailure('Second run stdout: $secondRunStdout');
-    printOnFailure('Second run stderr: ${secondRunResult.stderr.toString()}');
+    printOnFailure('Second run stderr: ${secondRunResult.stderr}');
 
     expect(secondRunResult.exitCode, 0);
     // Do not run "pod install" when nothing changes.