Small refactor

Author: mosuem

pkgs/hooks_runner/test_data/flag_enthusiast_1/hook/link.dart

@@ -9,6 +9,6 @@ import 'package:hooks/hooks.dart';
 // it lands on stable https://github.com/dart-lang/sdk/issues/61166.
 void main(List<String> args) async {
   await link(args, (input, output) async {
-    output.registerFlagUse(input.packageName, ['de']);
+    output.registerCountryUse(input.packageName, ['de']);
   });
 }

pkgs/hooks_runner/test_data/flag_enthusiast_2/hook/link.dart

@@ -9,6 +9,6 @@ import 'package:hooks/hooks.dart';
 // it lands on stable https://github.com/dart-lang/sdk/issues/61166.
 void main(List<String> args) async {
   await link(args, (input, output) async {
-    output.registerFlagUse(input.packageName, ['fr']);
+    output.registerCountryUse(input.packageName, ['fr']);
   });
 }

pkgs/hooks_runner/test_data/fun_with_flags/hook/link.dart

@@ -9,10 +9,7 @@ import 'package:hooks/hooks.dart';
 void main(List<String> args) {
   link(args, (input, output) async {
     print('Metadata: ${input.metadata}');
-    final usedFlags = input.metadata.entries
-        .where((entry) => entry.key.startsWith(prefix))
-        .expand((e) => e.value as List)
-        .map((e) => e as String)
+    final usedFlags = input.fetchUsedCountries
         .map((country) => 'package:fun_with_flags/assets/$country.txt')
         .toSet();
 

pkgs/hooks_runner/test_data/fun_with_flags/lib/hook.dart

@@ -2,4 +2,4 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-export 'src/hook.dart' show FlagsUsed;
+export 'src/hook.dart' show FlagsUsedWrite;

pkgs/hooks_runner/test_data/fun_with_flags/lib/src/hook.dart

@@ -2,16 +2,23 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:hooks/hooks.dart' show LinkOutputBuilder;
+import 'package:hooks/hooks.dart' show LinkInput, LinkOutputBuilder;
 
-const prefix = 'used_flags_';
+const _prefix = 'used_flags_';
 
-extension FlagsUsed on LinkOutputBuilder {
+extension FlagsUsedWrite on LinkOutputBuilder {
   /// Register a list of flags to be kept from treeshaking.
   ///
-  /// This uses the [callerPackageName] with the [prefix] to make a unique
+  /// This uses the [callerPackageName] with the [_prefix] to make a unique
   /// identifier for this list - that's how we avoid overwriting other
   /// packages flags.
-  void registerFlagUse(String callerPackageName, List<String> countries) =>
-      metadata.add('fun_with_flags', '$prefix$callerPackageName', countries);
+  void registerCountryUse(String callerPackageName, List<String> countries) =>
+      metadata.add('fun_with_flags', '$_prefix$callerPackageName', countries);
+}
+
+extension FlagsUsedRead on LinkInput {
+  Iterable<String> get fetchUsedCountries => metadata.entries
+      .where((entry) => entry.key.startsWith(_prefix))
+      .expand((e) => e.value as List)
+      .map((e) => e as String);
 }