diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index 5decfbdef895..795e88dd779d 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -4200,6 +4200,14 @@ GetPropertyHelperInline(JSContext *cx,
             if (!script || script->warnedAboutUndefinedProp())
                 return true;
 
+            /*
+             * Don't warn in self-hosted code (where the further presence of
+             * JS::ContextOptions::werror() would result in impossible-to-avoid
+             * errors to entirely-innocent client code).
+             */
+            if (script->selfHosted())
+                return true;
+
             /* We may just be checking if that object has an iterator. */
             if (JSID_IS_ATOM(id, cx->names().iteratorIntrinsic))
                 return true;
diff --git a/js/src/tests/Intl/extensions/browser.js b/js/src/tests/Intl/extensions/browser.js
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/js/src/tests/Intl/extensions/options-value-emulates-undefined.js b/js/src/tests/Intl/extensions/options-value-emulates-undefined.js
new file mode 100644
index 000000000000..e7af6a4860de
--- /dev/null
+++ b/js/src/tests/Intl/extensions/options-value-emulates-undefined.js
@@ -0,0 +1,29 @@
+// |reftest| skip-if(!xulRuntime.shell)
+// Any copyright is dedicated to the Public Domain.
+// http://creativecommons.org/licenses/publicdomain/
+
+//-----------------------------------------------------------------------------
+var BUGNUMBER = 843004;
+var summary =
+  "Use of an object that emulates |undefined| as the sole option must " +
+  "preclude imputing default values";
+
+print(BUGNUMBER + ": " + summary);
+
+/**************
+ * BEGIN TEST *
+ **************/
+
+var opt = objectEmulatingUndefined();
+opt.toString = function() { return "long"; };
+
+var str = new Date(2013, 12 - 1, 14).toLocaleString("en-US", { weekday: opt });
+
+// Because "weekday" was present and not undefined (stringifying to "long"),
+// this must be a string like "Saturday" (in this implementation, that is).
+assertEq(str, "Saturday");
+
+if (typeof reportCompare === "function")
+  reportCompare(true, true);
+
+print("Tests complete");
diff --git a/js/src/tests/Intl/extensions/shell.js b/js/src/tests/Intl/extensions/shell.js
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/js/src/tests/js1_8_5/extensions/selfhosted-detecting-strict.js b/js/src/tests/js1_8_5/extensions/selfhosted-detecting-strict.js
new file mode 100644
index 000000000000..cb4c026eaa83
--- /dev/null
+++ b/js/src/tests/js1_8_5/extensions/selfhosted-detecting-strict.js
@@ -0,0 +1,27 @@
+// Any copyright is dedicated to the Public Domain.
+// http://creativecommons.org/licenses/publicdomain/
+
+//-----------------------------------------------------------------------------
+var BUGNUMBER = 843004;
+var summary =
+  "Don't emit a strict warning for the undefined-property detection pattern in self-hosted code";
+
+print(BUGNUMBER + ": " + summary);
+
+/**************
+ * BEGIN TEST *
+ **************/
+
+options("strict", "werror");
+
+// Don't strict-warn (and throw, because of strict) when self-hosted code uses
+// detecting-safe undefined-property accesses (|options.weekday !== undefined|
+// and similar in ToDateTimeOptions, to be precise).
+new Date().toLocaleString("en-US", {});
+
+// If we get here, the test passed.
+
+if (typeof reportCompare === "function")
+  reportCompare(true, true);
+
+print("Tests complete");