Make Fuchsia tools Dart 2 clean (#19727)

diff --git a/packages/flutter_tools/lib/src/context_runner.dart b/packages/flutter_tools/lib/src/context_runner.dart
index cdc640f..c8cc501 100644
--- a/packages/flutter_tools/lib/src/context_runner.dart
+++ b/packages/flutter_tools/lib/src/context_runner.dart
@@ -37,7 +37,7 @@
 
 Future<T> runInContext<T>(
   FutureOr<T> runner(), {
-  Map<Type, dynamic> overrides,
+  Map<Type, Generator> overrides,
 }) async {
   return await context.run<T>(
     name: 'global fallbacks',