add missing type parameter on methods (#22096)

packages/flutter_tools/lib/src/base/utils.dart

diff --git a/packages/flutter_tools/lib/src/base/utils.dart b/packages/flutter_tools/lib/src/base/utils.dart
index 102a0ec..ee485de 100644
--- a/packages/flutter_tools/lib/src/base/utils.dart
+++ b/packages/flutter_tools/lib/src/base/utils.dart
@@ -202,7 +202,7 @@
   final Map<String, String> values = <String, String>{};
 
   void writeContents(File file) {
-    file.writeAsStringSync(values.keys.map((String key) {
+    file.writeAsStringSync(values.keys.map<String>((String key) {
       return '$key=${values[key]}';
     }).join('\n'));
   }