Expose Artifact#isTreeArtifact to Skylark.

--
MOS_MIGRATED_REVID=138112581

src/main/java/com/google/devtools/build/lib/actions/Artifact.java

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index 80d060d6..c2eedab 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -377,6 +377,8 @@
   /**
    * Returns true iff this is a TreeArtifact representing a directory tree containing Artifacts.
    */
+  // TODO(rduan): Document this Skylark method once TreeArtifact is no longer experimental.
+  @SkylarkCallable(name = "is_directory", structField = true, documented = false)
   public boolean isTreeArtifact() {
     return false;
   }