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

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);