url: define @@toStringTag as a data property

Even though this is not fully Web IDL spec-compliant, it is arguably the
best we can do. Following the spec would mean non-trivial performance
deterioration (10% when parsing a medium-length URL), while the current
getter behavior is not adopted by any implementer, and it causes some
spec ambiguity when the getter is called with !(this instanceof URL).

This commit adopts Chrome's behavior, and is consistent with
ECMAScript-defined classes while providing reasonable behaviors for
corner cases as well. Until the Web IDL spec is changed one way or
another, this is the way to go.

PR-URL: #10906
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

lib/internal/url.js

@@ -223,10 +223,6 @@ 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;
   }
@@ -314,6 +310,10 @@ Object.defineProperties(URL.prototype, {
       return ret;
     }
   },
+  [Symbol.toStringTag]: {
+    configurable: true,
+    value: 'URL'
+  },
   href: {
     enumerable: true,
     configurable: true,

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

@@ -45,7 +45,7 @@ assert.strictEqual(url.searchParams, oldParams);  // [SameObject]
 // Note: this error message is subject to change in V8 updates
 assert.throws(() => url.origin = 'http://foo.bar.com:22',
               new RegExp('TypeError: Cannot set property origin of' +
-                         ' \\[object Object\\] which has only a getter'));
+                         ' \\[object URL\\] which has only a getter'));
 assert.strictEqual(url.origin, 'http://foo.bar.com:21');
 assert.strictEqual(url.toString(),
                    'http://user:pass@foo.bar.com:21/aaa/zzz?l=25#test');
@@ -121,7 +121,7 @@ assert.strictEqual(url.hash, '#abcd');
 // Note: this error message is subject to change in V8 updates
 assert.throws(() => url.searchParams = '?k=88',
               new RegExp('TypeError: Cannot set property searchParams of' +
-                         ' \\[object Object\\] which has only a getter'));
+                         ' \\[object URL\\] which has only a getter'));
 assert.strictEqual(url.searchParams, oldParams);
 assert.strictEqual(url.toString(),
                    'https://user2:pass2@foo.bar.org:23/aaa/bbb?k=99#abcd');

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

@@ -10,9 +10,12 @@ 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']
+  [toString.call(Object.getPrototypeOf(sp)), 'URLSearchParamsPrototype'],
+  // Web IDL spec says we have to return 'URLPrototype', but it is too
+  // expensive to implement; therefore, use Chrome's behavior for now, until
+  // spec is changed.
+  [toString.call(Object.getPrototypeOf(url)), 'URL']
 ];
 
 test.forEach((row) => {