Documentation to follow up on CLs 714843002 and 719443002.

dart/pkg/dart2js_incremental/lib/library_updater.dart

Index: dart/pkg/dart2js_incremental/lib/library_updater.dart
diff --git a/dart/pkg/dart2js_incremental/lib/library_updater.dart b/dart/pkg/dart2js_incremental/lib/library_updater.dart
index 532b5840ee166302c263819fed6e90f298d8fe8d..7a8bbe893a367623c4e3f1ada527b87396413855 100644
--- a/dart/pkg/dart2js_incremental/lib/library_updater.dart
+++ b/dart/pkg/dart2js_incremental/lib/library_updater.dart
@@ -349,6 +349,9 @@ class LibraryUpdater extends JsFeatures {
         ' ${before} (${before.runtimeType} -> ${after.runtimeType}).');
   }
 
+  /// Apply the collected [updates]. Return a list of elements that needs to be
+  /// recompiled after applying the updates. Any elements removed as a
+  /// consequence of applying the patches are added to [removals] if provided.
   List<Element> applyUpdates([List<Update> removals]) {
     for (Update update in updates) {
       update.captureState();