apply upcomming prefer_const_declarations (#15498)

dev/bots/prepare_package.dart

diff --git a/dev/bots/prepare_package.dart b/dev/bots/prepare_package.dart
index 4622b04..9935908 100644
--- a/dev/bots/prepare_package.dart
+++ b/dev/bots/prepare_package.dart
@@ -480,7 +480,7 @@
     jsonData['releases'][revision][branchName] = metadata;
 
     final File tempFile = new File(path.join(tempDir.absolute.path, 'releases_$platformName.json'));
-    final JsonEncoder encoder = const JsonEncoder.withIndent('  ');
+    const JsonEncoder encoder = const JsonEncoder.withIndent('  ');
     tempFile.writeAsStringSync(encoder.convert(jsonData));
     await _cloudCopy(tempFile.absolute.path, metadataGsPath);
   }

dev/bots/test/prepare_package_test.dart

diff --git a/dev/bots/test/prepare_package_test.dart b/dev/bots/test/prepare_package_test.dart
index c460e7b..91e9405 100644
--- a/dev/bots/test/prepare_package_test.dart
+++ b/dev/bots/test/prepare_package_test.dart
@@ -16,7 +16,7 @@
 import 'fake_process_manager.dart';
 
 void main() {
-  final String testRef = 'deadbeefdeadbeefdeadbeefdeadbeefdeadbeef';
+  const String testRef = 'deadbeefdeadbeefdeadbeefdeadbeefdeadbeef';
   test('Throws on missing executable', () async {
     // Uses a *real* process manager, since we want to know what happens if
     // it can't find an executable.
@@ -295,7 +295,7 @@
         expect(contents, contains('"beta": "6da8ec6bd0c4801b80d666869e4069698561c043"'));
         // Make sure it's valid JSON, and in the right format.
         final Map<String, dynamic> jsonData = json.decode(contents);
-        final JsonEncoder encoder = const JsonEncoder.withIndent('  ');
+        const JsonEncoder encoder = const JsonEncoder.withIndent('  ');
         expect(contents, equals(encoder.convert(jsonData)));
       });
     });