Avoid delete in favor of null assignment to prevent deoptimizations

lib/6to5/transformation/transformers/es6-rest-parameters.js

@@ -5,7 +5,7 @@ exports.Function = function (node, parent, file) {
   if (!node.rest) return;
 
   var rest = node.rest;
-  delete node.rest;
+  node.rest = null;
 
   t.ensureBlock(node);
 

lib/6to5/traverse/index.js

@@ -102,7 +102,6 @@ function traverse(parent, opts, scope) {
     return;
   }
 
-  // unknown node type to traverse
   var keys = t.VISITOR_KEYS[parent.type];
   if (!keys) return;
 
@@ -139,15 +138,15 @@ function traverse(parent, opts, scope) {
 
 traverse.removeProperties = function (tree) {
   var clear = function (node) {
-    delete node._declarations;
-    delete node.extendedRange;
-    delete node._scopeInfo;
-    delete node.tokens;
-    delete node.range;
-    delete node.start;
-    delete node.end;
-    delete node.loc;
-    delete node.raw;
+    node._declarations = null;
+    node.extendedRange = null;
+    node._scopeInfo = null;
+    node.tokens = null;
+    node.range = null;
+    node.start = null;
+    node.end = null;
+    node.loc = null;
+    node.raw = null;
 
     clearComments(node.trailingComments);
     clearComments(node.leadingComments);