Declare locals final where not reassigned (flutter_tools) (#8570)

diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index 5facec6..8c0bcc5 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -130,7 +130,7 @@
   if (context == null || config == null)
     return 'build';
 
-  String buildDir = config.getValue('build-dir') ?? 'build';
+  final String buildDir = config.getValue('build-dir') ?? 'build';
   if (fs.path.isAbsolute(buildDir)) {
     throw new Exception(
         'build-dir config setting in ${config.configPath} must be relative');