add missing type parameter on methods (#22096)

packages/flutter_tools/lib/src/flutter_manifest.dart

diff --git a/packages/flutter_tools/lib/src/flutter_manifest.dart b/packages/flutter_tools/lib/src/flutter_manifest.dart
index 9e937f4..6fa8efb 100644
--- a/packages/flutter_tools/lib/src/flutter_manifest.dart
+++ b/packages/flutter_tools/lib/src/flutter_manifest.dart
@@ -224,7 +224,7 @@
   Map<String, dynamic> get descriptor {
     return <String, dynamic>{
       'family': familyName,
-      'fonts': fontAssets.map((FontAsset a) => a.descriptor).toList(),
+      'fonts': fontAssets.map<Map<String, dynamic>>((FontAsset a) => a.descriptor).toList(),
     };
   }