url: set toStringTag for the URL class

lib/internal/url.js

@@ -221,6 +221,10 @@ class URL {
     parse(this, input, base);
   }
 
+  get [Symbol.toStringTag]() {
+    return this instanceof URL ? 'URL' : 'URLPrototype';
+  }
+
   get [special]() {
     return (this[context].flags & binding.URL_FLAGS_SPECIAL) !== 0;
   }
@@ -640,15 +644,11 @@ class URLSearchParams {
 
     // "associated url object"
     this[context] = null;
+  }
 
-    // Class string for an instance of URLSearchParams. This is different from
-    // the class string of the prototype object (set below).
-    Object.defineProperty(this, Symbol.toStringTag, {
-      value: 'URLSearchParams',
-      writable: false,
-      enumerable: false,
-      configurable: true
-    });
+  get [Symbol.toStringTag]() {
+    return this instanceof URLSearchParams ?
+      'URLSearchParams' : 'URLSearchParamsPrototype';
   }
 
   append(name, value) {
@@ -803,12 +803,6 @@ class URLSearchParams {
 }
 // https://heycam.github.io/webidl/#es-iterable-entries
 URLSearchParams.prototype[Symbol.iterator] = URLSearchParams.prototype.entries;
-Object.defineProperty(URLSearchParams.prototype, Symbol.toStringTag, {
-  value: 'URLSearchParamsPrototype',
-  writable: false,
-  enumerable: false,
-  configurable: true
-});
 
 // https://heycam.github.io/webidl/#dfn-default-iterator-object
 function createSearchParamsIterator(target, kind) {

test/parallel/test-whatwg-url-tostringtag.js

@@ -0,0 +1,21 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const URL = require('url').URL;
+const toString = Object.prototype.toString;
+
+const url = new URL('http://example.org');
+const sp = url.searchParams;
+
+const test = [
+  [toString.call(url), 'URL'],
+  [toString.call(Object.getPrototypeOf(url)), 'URLPrototype'],
+  [toString.call(sp), 'URLSearchParams'],
+  [toString.call(Object.getPrototypeOf(sp)), 'URLSearchParamsPrototype']
+];
+
+test.forEach((row) => {
+  assert.strictEqual(row[0], `[object ${row[1]}]`,
+                     `${row[0]} !== [object ${row[1]}]`);
+});