[regexp] implement RegExp.prototype.toString for non-RegExp receiver.

R=littledan@chromium.org, verwaest@chromium.org
BUG=v8:4524
LOG=N

Review URL: https://codereview.chromium.org/1679123007

Cr-Commit-Position: refs/heads/master@{#33862}

src/js/regexp.js

@@ -278,8 +278,11 @@ function RegExpToString() {
       %IncrementUseCounter(kRegExpPrototypeToString);
       return '/(?:)/';
     }
-    throw MakeTypeError(kIncompatibleMethodReceiver,
-                        'RegExp.prototype.toString', this);
+    if (!IS_RECEIVER(this)) {
+      throw MakeTypeError(
+          kIncompatibleMethodReceiver, 'RegExp.prototype.toString', this);
+    }
+    return '/' + TO_STRING(this.pattern()) + '/' + TO_STRING(this.flags());
   }
   var result = '/' + REGEXP_SOURCE(this) + '/';
   if (REGEXP_GLOBAL(this)) result += 'g';

test/mjsunit/es6/regexp-tostring.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var log = [];
+
+var fake =
+    {
+      pattern: function() {
+        log.push("p");
+        return {
+          toString: function() {
+            log.push("ps");
+            return "pattern";
+          }
+        };
+      },
+      flags: function() {
+        log.push("f");
+        return {
+          toString: function() {
+            log.push("fs");
+            return "flags";
+          }
+        };
+      }
+    }
+
+function testThrows(x) {
+  try {
+    RegExp.prototype.toString.call(x);
+  } catch (e) {
+    assertTrue(/incompatible receiver/.test(e.message));
+    return;
+  }
+  assertUnreachable();
+}
+
+testThrows(1);
+testThrows(null);
+Number.prototype.pattern = () => "a";
+Number.prototype.flags = () => "b";
+testThrows(1);
+
+assertEquals("/pattern/flags", RegExp.prototype.toString.call(fake));
+assertEquals(["p", "ps", "f", "fs"], log);