Improve SyntaxTreeVisitor API.

RELNOTES: None.
PiperOrigin-RevId: 164481927
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Parameter.java b/src/main/java/com/google/devtools/build/lib/syntax/Parameter.java
index 29e122c..e16fe58 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Parameter.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Parameter.java
@@ -199,6 +199,6 @@
 
   @Override
   public void accept(SyntaxTreeVisitor visitor) {
-    visitor.visit(this);
+    visitor.visit((Parameter<Expression, Expression>) this);
   }
 }