--- a/dom/bindings/test/test_sequence_detection.html
+++ b/dom/bindings/test/test_sequence_detection.html
@@ -10,29 +10,41 @@ https://bugzilla.mozilla.org/show_bug.cg
   <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
   <script type="application/javascript">
 
   /** Test for Bug 1066432 **/
   SimpleTest.waitForExplicitFinish();
   SpecialPowers.pushPrefEnv({set: [['dom.expose_test_interfaces', true]]}, function() {
     var testInterfaceJS = new TestInterfaceJS();
     ok(testInterfaceJS, "got a TestInterfaceJS object");
+
+    var nonIterableObject = { "@@iterator": 5 };
+    if (typeof Symbol === "function") {
+      // Make this test fail if Symbol.iterator is correctly implemented.
+      // This is here to make sure this test is updated when bug 918828 lands,
+      // at which point the correct code will be:
+      //   var JS_HAS_SYMBOLS = typeof Symbol === "function";
+      //   var std_iterator = JS_HAS_SYMBOLS ? Symbol.iterator : "@@iterator";
+      //   var nonIterableObject = { [std_iterator]: 5};
+      // Otherwise, fixing bug 918828 would cause this test to silently stop
+      // testing what it's supposed to be testing.
+      nonIterableObject[Symbol.iterator] = Array.prototype[Symbol.iterator];
+    }
+
     try {
-      testInterfaceJS.testSequenceOverload(
-        { "@@iterator": 5, [Symbol.iterator]: Array.prototype[Symbol.iterator] });
-      ok(false, "Should have thrown in the overload case");
+      testInterfaceJS.testSequenceOverload(nonIterableObject);
+      ok(false, "Should have thrown in the overload case");  // see long comment above!
     } catch (e) {
       ise(e.name, "TypeError", "Should get a TypeError for the overload case");
       ok(e.message.contains("not iterable"),
          "Should have a message about being non-iterable in the overload case");
     }
 
     try {
-      testInterfaceJS.testSequenceUnion(
-        { "@@iterator": 5, [Symbol.iterator]: Array.prototype[Symbol.iterator] });
+      testInterfaceJS.testSequenceUnion(nonIterableObject);
       ok(false, "Should have thrown in the union case");
     } catch (e) {
       ise(e.name, "TypeError", "Should get a TypeError for the union case");
       ok(e.message.contains("not iterable"),
          "Should have a message about being non-iterable in the union case");
     }
 
     SimpleTest.finish();

--- a/js/src/jsapi-tests/testForOfIterator.cpp
+++ b/js/src/jsapi-tests/testForOfIterator.cpp
@@ -2,37 +2,47 @@
  * vim: set ts=8 sts=4 et sw=4 tw=99:
  */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "jsapi-tests/tests.h"
 
+#ifdef JS_HAS_SYMBOLS
+#define IF_JS_HAS_SYMBOLS(x) x
+#else
+#define IF_JS_HAS_SYMBOLS(x)
+#endif
+
 BEGIN_TEST(testForOfIterator_basicNonIterable)
 {
     JS::RootedValue v(cx);
     // Hack to make it simple to produce an object that has a property
     // named Symbol.iterator.
-    EVAL("var obj = { '@@iterator': 5, [Symbol.iterator]: Array.prototype[Symbol.iterator] }; obj;", &v);
+    EVAL("var obj = {'@@iterator': 5"
+         IF_JS_HAS_SYMBOLS(", [Symbol.iterator]: Array.prototype[Symbol.iterator]")
+         "}; obj;", &v);
     JS::ForOfIterator iter(cx);
     bool ok = iter.init(v);
     CHECK(!ok);
     JS_ClearPendingException(cx);
     return true;
 }
 END_TEST(testForOfIterator_basicNonIterable)
 
 BEGIN_TEST(testForOfIterator_bug515273_part1)
 {
     JS::RootedValue v(cx);
 
     // Hack to make it simple to produce an object that has a property
     // named Symbol.iterator.
-    EVAL("var obj = { '@@iterator': 5, [Symbol.iterator]: Array.prototype[Symbol.iterator] }; obj;", &v);
+    EVAL("var obj = {'@@iterator': 5"
+         IF_JS_HAS_SYMBOLS(", [Symbol.iterator]: Array.prototype[Symbol.iterator]")
+         "}; obj;", &v);
 
     JS::ForOfIterator iter(cx);
     bool ok = iter.init(v, JS::ForOfIterator::AllowNonIterable);
     CHECK(!ok);
     JS_ClearPendingException(cx);
     return true;
 }
 END_TEST(testForOfIterator_bug515273_part1)