Introduce enum to quickly discriminate AST nodes.

RELNOTES: None.
PiperOrigin-RevId: 166370036
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SliceExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/SliceExpression.java
index 71b6376..a2b6036 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SliceExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SliceExpression.java
@@ -118,4 +118,9 @@
   public void accept(SyntaxTreeVisitor visitor) {
     visitor.visit(this);
   }
+
+  @Override
+  public Kind kind() {
+    return Kind.SLICE;
+  }
 }