Disable unsafePackageSerialization as it causes issues when coverage is turned on (#23280)

* Disable unsafePackageSerialization as it causes issues when coverage is
used (see https://github.com/dart-lang/sdk/issues/34841)

* Restore Coverage.

dev/bots/test.dart

@@ -194,28 +194,26 @@ Future<void> _runTests() async {
   print('${bold}DONE: All tests successful.$reset');
 }
 
-// TODO(yjbanov): re-enable coverage collection when https://github.com/dart-lang/sdk/issues/34841 is fixed.
 Future<void> _runCoverage() async {
-  print('${bold}DONE: Coverage collection SKIPPED due to https://github.com/dart-lang/sdk/issues/34841.$reset');
-  // final File coverageFile = File(path.join(flutterRoot, 'packages', 'flutter', 'coverage', 'lcov.info'));
-  // if (!coverageFile.existsSync()) {
-  //   print('${red}Coverage file not found.$reset');
-  //   print('Expected to find: ${coverageFile.absolute}');
-  //   print('This file is normally obtained by running `flutter update-packages`.');
-  //   exit(1);
-  // }
-  // coverageFile.deleteSync();
-  // await _runFlutterTest(path.join(flutterRoot, 'packages', 'flutter'),
-  //   options: const <String>['--coverage'],
-  // );
-  // if (!coverageFile.existsSync()) {
-  //   print('${red}Coverage file not found.$reset');
-  //   print('Expected to find: ${coverageFile.absolute}');
-  //   print('This file should have been generated by the `flutter test --coverage` script, but was not.');
-  //   exit(1);
-  // }
+  final File coverageFile = File(path.join(flutterRoot, 'packages', 'flutter', 'coverage', 'lcov.info'));
+  if (!coverageFile.existsSync()) {
+    print('${red}Coverage file not found.$reset');
+    print('Expected to find: ${coverageFile.absolute}');
+    print('This file is normally obtained by running `flutter update-packages`.');
+    exit(1);
+  }
+  coverageFile.deleteSync();
+  await _runFlutterTest(path.join(flutterRoot, 'packages', 'flutter'),
+    options: const <String>['--coverage'],
+  );
+  if (!coverageFile.existsSync()) {
+    print('${red}Coverage file not found.$reset');
+    print('Expected to find: ${coverageFile.absolute}');
+    print('This file should have been generated by the `flutter test --coverage` script, but was not.');
+    exit(1);
+  }
 
-  // print('${bold}DONE: Coverage collection successful.$reset');
+  print('${bold}DONE: Coverage collection successful.$reset');
 }
 
 Future<void> _pubRunTest(

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -239,7 +239,7 @@ class _Compiler {
         trackWidgetCreation: trackWidgetCreation,
         compilerMessageConsumer: reportCompilerMessage,
         initializeFromDill: testFilePath,
-        unsafePackageSerialization: true,
+        unsafePackageSerialization: false,
       );
     }