Introduce enum to quickly discriminate AST nodes.

RELNOTES: None.
PiperOrigin-RevId: 166370036
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/ConditionalExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/ConditionalExpression.java
index c2d1c5d..e5a0b4c 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/ConditionalExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/ConditionalExpression.java
@@ -65,4 +65,9 @@
   public void accept(SyntaxTreeVisitor visitor) {
     visitor.visit(this);
   }
+
+  @Override
+  public Kind kind() {
+    return Kind.CONDITIONAL;
+  }
 }