Skylark: Allow list slices

--
MOS_MIGRATED_REVID=88727892
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
index c31d434..41fe332 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/FuncallExpression.java
@@ -247,7 +247,7 @@
 
   @Override
   public String toString() {
-    if (func.getName().equals("$substring")) {
+    if (func.getName().equals("$slice")) {
       return obj + "[" + args.get(0) + ":" + args.get(1) + "]";
     }
     if (func.getName().equals("$index")) {