workaround toString() quirks on global context (#4814)

test/sandbox.js

@@ -170,12 +170,6 @@ function setup(global, builtins, setup_log, setup_tty) {
         },
         global: { get: self },
         self: { get: self },
-        // for Node.js v8+
-        toString: {
-            get: function() {
-                return global_toString;
-            },
-        },
         window: { get: self },
     };
     [
@@ -205,15 +199,19 @@ function setup(global, builtins, setup_log, setup_tty) {
         } catch (e) {}
     });
     Object.defineProperties(global, props);
+    // for Node.js v8+
+    if (global.toString !== Object.prototype.toString) {
+        global.__proto__ = Object.defineProperty(Object.create(global.__proto__), "toString", {
+            value: function() {
+                return "[object global]";
+            },
+        });
+    }
 
     function self() {
         return this;
     }
 
-    function global_toString() {
-        return "[object global]";
-    }
-
     function safe_log(arg, cache) {
         if (arg) switch (typeof arg) {
           case "function":