Introduce TreeFileArtifact, which represents files under TreeArtifacts.
Remove ArtifactFile, which is rendered obsolete by TreeFileArtifact.

--
MOS_MIGRATED_REVID=119789154
diff --git a/src/test/java/com/google/devtools/build/lib/actions/ArtifactTest.java b/src/test/java/com/google/devtools/build/lib/actions/ArtifactTest.java
index 37f57c8..ae2d09a 100644
--- a/src/test/java/com/google/devtools/build/lib/actions/ArtifactTest.java
+++ b/src/test/java/com/google/devtools/build/lib/actions/ArtifactTest.java
@@ -201,7 +201,7 @@
 
   @Test
   public void testAddExpandedArtifacts() throws Exception {
-    List<ArtifactFile> expanded = new ArrayList<>();
+    List<Artifact> expanded = new ArrayList<>();
     MutableActionGraph actionGraph = new MapBasedActionGraph();
     List<Artifact> original = getFooBarArtifacts(actionGraph, true);
     Artifact.addExpandedArtifacts(original, expanded,
@@ -252,7 +252,7 @@
   // TODO consider tests for the future
   @Test
   public void testAddExpandedArtifactsNewActionGraph() throws Exception {
-    List<ArtifactFile> expanded = new ArrayList<>();
+    List<Artifact> expanded = new ArrayList<>();
     MutableActionGraph actionGraph = new MapBasedActionGraph();
     List<Artifact> original = getFooBarArtifacts(actionGraph, true);
     Artifact.addExpandedArtifacts(original, expanded,