new file mode 100644
--- /dev/null
+++ b/mfbt/Assertions.h
@@ -0,0 +1,83 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sw=4 et tw=99 ft=cpp:
+ *
+ * ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at:
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is Mozilla Code.
+ *
+ * The Initial Developer of the Original Code is
+ *   The Mozilla Foundation
+ * Portions created by the Initial Developer are Copyright (C) 2011
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Jeff Walden <jwalden+code@mit.edu>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#ifndef mozilla_Assertions_h_
+#define mozilla_Assertions_h_
+
+#include "mozilla/Types.h"
+
+/*
+ * XXX: we're cheating here in order to avoid creating object files
+ * for mfbt /just/ to provide a function like FatalError() to be used
+ * by MOZ_ASSERT().  (It'll happen eventually, but for just ASSERT()
+ * it isn't worth the pain.)  JS_Assert(), although unfortunately
+ * named, is part of SpiderMonkey's stable, external API, so this
+ * isn't quite as bad as it seems.
+ *
+ * Once mfbt needs object files, this unholy union with JS_Assert()
+ * will be broken.
+ *
+ * JS_Assert is present even in release builds, for the benefit of applications
+ * that build DEBUG and link against a non-DEBUG SpiderMonkey library.
+ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern MFBT_API(void)
+JS_Assert(const char* s, const char* file, int ln);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+/*
+ * MOZ_ASSERT() is a "strong" assertion of state, like libc's
+ * assert().  If a MOZ_ASSERT() fails in a debug build, the process in
+ * which it fails will stop running in a loud and dramatic way.
+ */
+#ifdef DEBUG
+#  define MOZ_ASSERT(expr_)                                      \
+     ((expr_) ? ((void)0) : JS_Assert(#expr_, __FILE__, __LINE__))
+#else
+#  define MOZ_ASSERT(expr_) ((void)0)
+#endif /* DEBUG */
+
+#endif /* mozilla_Assertions_h_ */

--- a/mfbt/GuardObjects.h
+++ b/mfbt/GuardObjects.h
@@ -36,18 +36,17 @@
  * the provisions above, a recipient may use your version of this file under
  * the terms of any one of the MPL, the GPL or the LGPL.
  *
  * ***** END LICENSE BLOCK ***** */
 
 #ifndef mozilla_GuardObjects_h
 #define mozilla_GuardObjects_h
 
-#include "mozilla/Types.h"
-#include "mozilla/Util.h"
+#include "mozilla/Assertions.h"
 
 namespace mozilla {
   /**
    * The following classes are designed to cause assertions to detect
    * inadvertent use of guard objects as temporaries.  In other words,
    * when we have a guard object whose only purpose is its constructor and
    * destructor (and is never otherwise referenced), the intended use
    * might be:

--- a/mfbt/RangedPtr.h
+++ b/mfbt/RangedPtr.h
@@ -36,16 +36,17 @@
  * the provisions above, a recipient may use your version of this file under
  * the terms of any one of the MPL, the GPL or the LGPL.
  *
  * ***** END LICENSE BLOCK ***** */
 
 #ifndef mozilla_RangedPtr_h_
 #define mozilla_RangedPtr_h_
 
+#include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/Util.h"
 
 namespace mozilla {
 
 /*
  * RangedPtr is a smart pointer restricted to an address range specified at
  * creation.  The pointer (and any smart pointers derived from it) must remain

--- a/mfbt/RefPtr.h
+++ b/mfbt/RefPtr.h
@@ -36,16 +36,17 @@
  * the provisions above, a recipient may use your version of this file under
  * the terms of any one of the MPL, the GPL or the LGPL.
  *
  * ***** END LICENSE BLOCK ***** */
 
 #ifndef mozilla_RefPtr_h_
 #define mozilla_RefPtr_h_
 
+#include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/Util.h"
 
 /**
  * Helpers for defining and using refcounted objects.
  */
 
 namespace mozilla {

--- a/mfbt/Util.h
+++ b/mfbt/Util.h
@@ -35,53 +35,21 @@
  * the provisions above, a recipient may use your version of this file under
  * the terms of any one of the MPL, the GPL or the LGPL.
  *
  * ***** END LICENSE BLOCK ***** */
 
 #ifndef mozilla_Util_h_
 #define mozilla_Util_h_
 
+#include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/Types.h"
 
 /*
- * XXX: we're cheating here in order to avoid creating object files
- * for mfbt /just/ to provide a function like FatalError() to be used
- * by MOZ_ASSERT().  (It'll happen eventually, but for just ASSERT()
- * it isn't worth the pain.)  JS_Assert(), although unfortunately
- * named, is part of SpiderMonkey's stable, external API, so this
- * isn't quite as bad as it seems.
- *
- * Once mfbt needs object files, this unholy union with JS_Assert()
- * will be broken.
- *
- * JS_Assert is present even in release builds, for the benefit of applications
- * that build DEBUG and link against a non-DEBUG SpiderMonkey library.
- */
-MOZ_BEGIN_EXTERN_C
-
-extern MFBT_API(void)
-JS_Assert(const char *s, const char *file, int ln);
-
-MOZ_END_EXTERN_C
-
-/*
- * MOZ_ASSERT() is a "strong" assertion of state, like libc's
- * assert().  If a MOZ_ASSERT() fails in a debug build, the process in
- * which it fails will stop running in a loud and dramatic way.
- */
-#ifdef DEBUG
-#  define MOZ_ASSERT(expr_)                                      \
-     ((expr_) ? (void)0 : JS_Assert(#expr_, __FILE__, __LINE__))
-#else
-#  define MOZ_ASSERT(expr_) ((void)0)
-#endif  /* DEBUG */
-
-/*
  * MOZ_INLINE is a macro which expands to tell the compiler that the method
  * decorated with it should be inlined.  This macro is usable from C and C++
  * code, even though C89 does not support the |inline| keyword.  The compiler
  * may ignore this directive if it chooses.
  */
 #ifndef MOZ_INLINE
 #  if defined __cplusplus
 #    define MOZ_INLINE          inline

--- a/mfbt/exported_headers.mk
+++ b/mfbt/exported_headers.mk
@@ -37,16 +37,17 @@
 
 # This file defines the headers exported by mfbt.  It is included by mfbt
 # itself and by the JS engine, which, when built standalone, must install
 # mfbt's exported headers itself.
 
 EXPORTS_NAMESPACES += mozilla
 
 EXPORTS_mozilla += \
+  Assertions.h \
   Attributes.h \
   GuardObjects.h \
   MSStdInt.h \
   RangedPtr.h \
   RefPtr.h \
   StdInt.h \
   Types.h \
   Util.h \