Tighten the type of NodeEntry#getAllRemainingDirtyDirectDeps since it is always an ImmutableSet.

PiperOrigin-RevId: 241094312
diff --git a/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java b/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java
index 50073cd..1ead12f 100644
--- a/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java
+++ b/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java
@@ -612,7 +612,8 @@
   }
 
   @Override
-  public synchronized Set<SkyKey> getAllRemainingDirtyDirectDeps() throws InterruptedException {
+  public synchronized ImmutableSet<SkyKey> getAllRemainingDirtyDirectDeps()
+      throws InterruptedException {
     Preconditions.checkNotNull(dirtyBuildingState, this);
     Preconditions.checkState(
         dirtyBuildingState.isEvaluating(), "Not evaluating for remaining dirty? %s", this);