re-enable lint unnecessary_const (#19592)

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)
diff --git a/packages/flutter_tools/lib/src/project.dart b/packages/flutter_tools/lib/src/project.dart
index ad506d2..bda7ede 100644
--- a/packages/flutter_tools/lib/src/project.dart
+++ b/packages/flutter_tools/lib/src/project.dart
@@ -219,7 +219,7 @@
   bool isUsingGradle() {
     return directory.childFile('build.gradle').existsSync();
   }
-  
+
   final Directory directory;
 
   Future<String> applicationId() {