workaround v8 performance quirks (#5207)

lib/scope.js

@@ -45,6 +45,7 @@
 
 function SymbolDef(id, scope, orig, init) {
     this._bits = 0;
+    this.defun = undefined;
     this.eliminated = 0;
     this.id = id;
     this.init = init;
@@ -53,6 +54,7 @@ function SymbolDef(id, scope, orig, init) {
     this.orig = [ orig ];
     this.references = [];
     this.replaced = 0;
+    this.safe_ids = undefined;
     this.scope = scope;
 }
 
@@ -370,7 +372,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         if (node instanceof AST_SymbolLambda) {
             var def = node.thedef;
             if (!redefine(node, node.scope.parent_scope.resolve())) {
-                delete def.defun;
+                def.defun = undefined;
             } else if (typeof node.thedef.init !== "undefined") {
                 node.thedef.init = false;
             } else if (def.init) {
@@ -472,7 +474,7 @@ AST_Symbol.DEFMETHOD("mark_enclosed", function(options) {
     for (var s = this.scope; s; s = s.parent_scope) {
         push_uniq(s.enclosed, def);
         if (!options) {
-            delete s._var_names;
+            s._var_names = undefined;
         } else if (options.keep_fnames) {
             s.functions.each(function(d) {
                 push_uniq(def.scope.enclosed, d);