src/jdk.scripting.nashorn/share/classes/jdk/nashorn/internal/codegen/Lower.java

@@ -95,10 +95,11 @@
  */
 @Logger(name="lower")
 final class Lower extends NodeOperatorVisitor<BlockLexicalContext> implements Loggable {
 
     private final DebugLogger log;
+    private final boolean es6;
 
     // Conservative pattern to test if element names consist of characters valid for identifiers.
     // This matches any non-zero length alphanumeric string including _ and $ and not starting with a digit.
     private static final Pattern SAFE_PROPERTY_NAME = Pattern.compile("[a-zA-Z_$][\\w$]*");
 

@@ -142,10 +143,11 @@
                 return block.setIsTerminal(this, false);
             }
         });
 
         this.log = initLogger(compiler.getContext());
+        this.es6 = compiler.getScriptEnvironment()._es6;
     }
 
     @Override
     public DebugLogger getLogger() {
         return log;

@@ -255,12 +257,13 @@
         if (!forNode.isForIn() && isAlwaysTrue(test)) {
             newForNode = forNode.setTest(lc, null);
         }
 
         newForNode = checkEscape(newForNode);
-        if(newForNode.isForIn()) {
-            // Wrap it in a block so its internally created iterator is restricted in scope
+        if(!es6 && newForNode.isForIn()) {
+            // Wrap it in a block so its internally created iterator is restricted in scope, unless we are running
+            // in ES6 mode, in which case the parser already created a block to capture let/const declarations.
             addStatementEnclosedInBlock(newForNode);
         } else {
             addStatement(newForNode);
         }
         return newForNode;