Add file extension property to Skylark

RELNOTES[NEW]: Files now have an "extension" property in Skylark.

--
MOS_MIGRATED_REVID=136425934
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 e43ddd1..81b64b2 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
@@ -33,16 +33,14 @@
 import com.google.devtools.build.lib.testutil.Scratch;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
-
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
-import java.io.IOException;
-import java.util.ArrayList;
-import java.util.List;
-
 @RunWith(JUnit4.class)
 public class ArtifactTest {
   private Scratch scratch;
@@ -149,6 +147,13 @@
   }
 
   @Test
+  public void testGetExtension() throws Exception {
+    Root root = Root.asSourceRoot(scratch.dir("/foo"));
+    Artifact javaFile = new Artifact(scratch.file("/foo/Bar.java"), root);
+    assertThat(javaFile.getExtension()).isEqualTo("java");
+  }
+
+  @Test
   public void testMangledPath() {
     String path = "dir/sub_dir/name:end";
     assertEquals("dir_Ssub_Udir_Sname_Cend", Actions.escapedPath(path));