[record_use] Use operator + on Recordings instead of custom merge method (#191291)

(We added the method upstream.)

packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart

diff --git a/packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart b/packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart
index 4e66092..6439756 100644
--- a/packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart
+++ b/packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart
@@ -130,7 +130,7 @@
       final Recordings recordings = await _readRecordings(file);
       combinedRecordings = combinedRecordings == null
           ? recordings
-          : combinedRecordings.merge(recordings);
+          : combinedRecordings + recordings;
     }
 
     final Map<String, List<int>> iconData = _parseRecordings(combinedRecordings!);
@@ -479,30 +479,3 @@
       'To disable icon tree shaking, pass --no-tree-shake-icons to the requested '
       'flutter build command';
 }
-
-extension on Recordings {
-  /// Returns a new [Recordings] containing all usages from both `this` and
-  /// [other].
-  ///
-  /// If a definition is present in both recordings, its usages from both
-  /// are combined in the returned [Recordings].
-  Recordings merge(Recordings other) {
-    final newCalls = <DefinitionWithStaticCalls, List<CallReference>>{};
-    for (final MapEntry(:key, :value) in calls.entries) {
-      newCalls[key] = <CallReference>[...value];
-    }
-    for (final MapEntry(:key, :value) in other.calls.entries) {
-      newCalls.putIfAbsent(key, () => <CallReference>[]).addAll(value);
-    }
-
-    final newInstances = <DefinitionWithInstances, List<InstanceReference>>{};
-    for (final MapEntry(:key, :value) in instances.entries) {
-      newInstances[key] = <InstanceReference>[...value];
-    }
-    for (final MapEntry(:key, :value) in other.instances.entries) {
-      newInstances.putIfAbsent(key, () => <InstanceReference>[]).addAll(value);
-    }
-
-    return Recordings(calls: newCalls, instances: newInstances);
-  }
-}