Fix d8-os unit test to be skipped for isolates.

test/mjsunit/d8-os.js

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 78 matching lines @@
     assertThrows("os.mkdirp(TEST_DIR + '/file1');", "mkdir over file1");
     assertThrows("os.mkdirp(TEST_DIR + '/file1/foo');", "mkdir over file2");
     assertThrows("os.mkdirp(TEST_DIR + '/file1/');", "mkdir over file3");
     assertThrows("os.mkdirp(TEST_DIR + '/file1/foo/');", "mkdir over file4");
     // Create a dir we cannot read.
     os.mkdirp(TEST_DIR + "/dir4", 0);
     // This test fails if you are root since root can read any dir.
     assertThrows("os.chdir(TEST_DIR + '/dir4');", "chdir dir4 I");
     os.rmdir(TEST_DIR + "/dir4");
     assertThrows("os.chdir(TEST_DIR + '/dir4');", "chdir dir4 II");
-    // Set umask.
+
+    // Set umask.  This changes the umask for the whole process and is
+    // the reason why the test cannot be run multi-threaded.
     var old_umask = os.umask(0777);
     // Create a dir we cannot read.
     os.mkdirp(TEST_DIR + "/dir5");
     // This test fails if you are root since root can read any dir.
     assertThrows("os.chdir(TEST_DIR + '/dir5');", "cd dir5 I");
     os.rmdir(TEST_DIR + "/dir5");
     assertThrows("os.chdir(TEST_DIR + '/dir5');", "chdir dir5 II");
     os.umask(old_umask);
 
     os.mkdirp(TEST_DIR + "/hest/fisk/../fisk/ged");
@@ skipping 67 matching lines @@
     str_error("os.mkdirp(e);");
     str_error("os.setenv(e, 'goo');");
     str_error("os.setenv('goo', e);");
     str_error("os.chdir(e);");
     str_error("os.rmdir(e);");
 
   } finally {
     os.system("rm", ["-r", TEST_DIR]);
   }
 }