Allow tree artifacts to be omitted, take 2.

PiperOrigin-RevId: 314357168
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ActionInputMapHelper.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionInputMapHelper.java
index 9c7a570..03fed1a 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionInputMapHelper.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionInputMapHelper.java
@@ -157,6 +157,10 @@
       Map<Artifact, Collection<Artifact>> expandedArtifacts,
       ActionInputMapSink inputMap,
       Artifact depOwner) {
+    if (TreeArtifactValue.OMITTED_TREE_MARKER.equals(value)) {
+      inputMap.put(treeArtifact, FileArtifactValue.OMITTED_FILE_MARKER, depOwner);
+      return;
+    }
     ImmutableSet.Builder<Artifact> children = ImmutableSet.builder();
     for (Map.Entry<Artifact.TreeFileArtifact, FileArtifactValue> child :
         value.getChildValues().entrySet()) {