--- a/js/src/Makefile.in
+++ b/js/src/Makefile.in
@@ -551,23 +551,63 @@ ifndef MOZ_NATIVE_FFI
 export::
 		$(call SUBMAKE,,ctypes/libffi)
 
 distclean clean::
 		$(call SUBMAKE,$@,ctypes/libffi)
 endif
 endif
 
+
+#############################################
+# BEGIN ECMAScript Internationalization API
+#
+
 # ICU headers need to be available whether we build with the complete
 # Internationalization API or not - ICU stubs rely on them.
 
 LOCAL_INCLUDES += \
-    -I$(topsrcdir)/../../intl/icu/source/common \
-    -I$(topsrcdir)/../../intl/icu/source/i18n \
-    $(NULL)
+  -I$(topsrcdir)/../../intl/icu/source/common \
+  -I$(topsrcdir)/../../intl/icu/source/i18n \
+  $(NULL)
+
+ifdef ENABLE_INTL_API
+
+ifeq ($(OS_ARCH),WINNT)
+  # Parallel gmake is buggy on Windows
+  ICU_GMAKE_OPTIONS="-j1"
+  # Library names: On Windows, ICU uses modified library names for static
+  # and debug libraries.
+  ifdef MOZ_DEBUG
+    ICU_LIB_SUFFIX=d
+  endif
+  ICU_LIB_RENAME = $(foreach libname,$(ICU_LIB_NAMES),\
+                     cp -p intl/icu/lib/s$(libname)$(ICU_LIB_SUFFIX).lib intl/icu/lib/$(libname).lib;)
+endif
+
+# - Build ICU as part of the "export" target, so things get built
+#   in the right order.
+# - ICU requires GNU make according to its readme.html. pymake can't be used
+#   because it doesn't support order only dependencies.
+# - Force ICU to use the standard suffix for object files because expandlibs
+#   will discard all files with a non-standard suffix (bug 857450).
+# - Options for genrb: -k strict parsing; -R omit collation tailoring rules.
+export::
+	$(GMAKE) $(ICU_GMAKE_OPTIONS) -C intl/icu STATIC_O=$(OBJ_SUFFIX) GENRBOPTS='-k -R'
+	$(ICU_LIB_RENAME)
+
+distclean clean::
+	$(call SUBMAKE,$@,intl/icu)
+
+endif
+
+#
+# END ECMAScript Internationalization API
+#############################################
+
 
 # The "find any vanilla new/new[] calls" script is tailored to Linux, so
 # only run it there.  That should be enough to catch any such calls that
 # creep in.
 check-vanilla-new:
 	$(srcdir)/config/find_vanilla_new_calls $(LIBRARY)
 
 ifeq ($(OS_ARCH),Linux)
@@ -682,16 +722,18 @@ CFLAGS += $(MOZ_ZLIB_CFLAGS)
 EXTRA_LIBS += $(MOZ_ZLIB_LIBS)
 # Enable zlib usage if zlib has been located. When building the browser on
 # Windows, MOZ_ZLIB_LIBS is empty because zlib is part of libmozglue. We thus
 # also enable zlib if mozglue is present.
 ifneq (,$(MOZ_ZLIB_LIBS)$(MOZ_GLUE_LDFLAGS))
 DEFINES += -DUSE_ZLIB
 endif
 
+SHARED_LIBRARY_LIBS += $(ICU_LIBS)
+
 # Prevent floating point errors caused by VC++ optimizations
 ifdef _MSC_VER
 # XXX We should add this to CXXFLAGS, too?
 CFLAGS += -fp:precise
 
 ifeq ($(CPU_ARCH),x86)
 # Workaround compiler bug on PGO (Bug 721284)
 MonoIC.$(OBJ_SUFFIX): CXXFLAGS += -GL-
@@ -979,9 +1021,8 @@ endif
 
 TestMain$(HOST_BIN_SUFFIX): $(TESTMAIN_OBJS)
 	$(CXX) -o TestMain$(HOST_BIN_SUFFIX) $(TESTMAIN_OBJS)
 endif
 
 #
 # END kludges for the Nitro assembler
 ###############################################
-

--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -4398,24 +4398,112 @@ if test "$ACCESSIBILITY" -a "$MOZ_ENABLE
     ATK_MAJOR_VERSION=`echo ${ATK_FULL_VERSION} | $AWK -F\. '{ print $1 }'`
     ATK_MINOR_VERSION=`echo ${ATK_FULL_VERSION} | $AWK -F\. '{ print $2 }'`
     ATK_REV_VERSION=`echo ${ATK_FULL_VERSION} | $AWK -F\. '{ print $3 }'`
     AC_DEFINE_UNQUOTED(ATK_MAJOR_VERSION, $ATK_MAJOR_VERSION)
     AC_DEFINE_UNQUOTED(ATK_MINOR_VERSION, $ATK_MINOR_VERSION)
     AC_DEFINE_UNQUOTED(ATK_REV_VERSION, $ATK_REV_VERSION)
 fi
 
+
+dnl ========================================================
 dnl ECMAScript Internationalization API Support (uses ICU)
 dnl ========================================================
 
+MOZ_ARG_ENABLE_BOOL(intl-api,
+[  --enable-intl-api       Enable ECMAScript Internationalization API],
+    ENABLE_INTL_API=1 )
+
+dnl Settings for the implementation of the ECMAScript Internationalization API
+if test -n "$ENABLE_INTL_API"; then
+    AC_DEFINE(ENABLE_INTL_API)
+    # We build ICU as a static library.
+    AC_DEFINE(U_STATIC_IMPLEMENTATION)
+
+    case "$OS_TARGET" in
+        WINNT)
+            ICU_LIB_NAMES="icuin icuuc icudt"
+            ;;
+        Darwin|Linux)
+            ICU_LIB_NAMES="icui18n icuuc icudata"
+            ;;
+        *)
+            AC_MSG_ERROR([ECMAScript Internationalization API is not yet supported on this platform])
+    esac
+
+    ICU_LIBS='$(call EXPAND_LIBNAME_PATH,$(ICU_LIB_NAMES),$(DEPTH)/intl/icu/lib)'
+else
+    ICU_LIB_NAMES=
+    ICU_LIBS=
+fi
+
+AC_SUBST(ENABLE_INTL_API)
+AC_SUBST(ICU_LIB_NAMES)
+AC_SUBST(ICU_LIBS)
+
 dnl Source files that use ICU should have control over which parts of the ICU
 dnl namespace they want to use.
 AC_DEFINE(U_USING_ICU_NAMESPACE,0)
 
 
+dnl Settings for ICU
+if test -n "$ENABLE_INTL_API" ; then
+    # Set ICU compile options
+    ICU_CPPFLAGS=""
+    # don't use icu namespace automatically in client code
+    ICU_CPPFLAGS="$ICU_CPPFLAGS -DU_USING_ICU_NAMESPACE=0"
+    # don't include obsolete header files
+    ICU_CPPFLAGS="$ICU_CPPFLAGS -DU_NO_DEFAULT_INCLUDE_UTF_HEADERS=1"
+    # remove chunks of the library that we don't need (yet)
+    ICU_CPPFLAGS="$ICU_CPPFLAGS -DUCONFIG_NO_LEGACY_CONVERSION"
+    ICU_CPPFLAGS="$ICU_CPPFLAGS -DUCONFIG_NO_TRANSLITERATION"
+    ICU_CPPFLAGS="$ICU_CPPFLAGS -DUCONFIG_NO_REGULAR_EXPRESSIONS"
+    ICU_CPPFLAGS="$ICU_CPPFLAGS -DUCONFIG_NO_BREAK_ITERATION"
+
+    # Set OS dependent options for ICU
+    case "$OS_TARGET" in
+        Darwin)
+            ICU_TARGET=MacOSX
+            ;;
+        Linux)
+            ICU_TARGET=Linux
+            ;;
+        WINNT)
+            ICU_TARGET=MSYS/MSVC
+            ;;
+    esac
+
+    # To reduce library size, use static linking
+    ICU_LINK_OPTS="--enable-static --disable-shared"
+    # Force the ICU static libraries to be position independent code
+    ICU_CFLAGS="$DSO_PIC_CFLAGS"
+    ICU_CXXFLAGS="$DSO_PIC_CFLAGS"
+
+    ICU_BUILD_OPTS=""
+    if test -n "$MOZ_DEBUG"; then
+        ICU_BUILD_OPTS="$ICU_BUILD_OPTS --enable-debug"
+    fi
+    if test -z "$MOZ_OPTIMIZE"; then
+        ICU_BUILD_OPTS="$ICU_BUILD_OPTS --disable-release"
+    fi
+
+    abs_srcdir=`(cd $srcdir; pwd)`
+    mkdir -p $_objdir/intl/icu
+    (cd $_objdir/intl/icu; \
+     CFLAGS="$ICU_CFLAGS" CPPFLAGS="$ICU_CPPFLAGS" CXXFLAGS="$ICU_CXXFLAGS" \
+            $(SHELL) $abs_srcdir/../../intl/icu/source/runConfigureICU \
+            $ICU_BUILD_OPTS \
+            $ICU_TARGET \
+            $ICU_LINK_OPTS \
+            --enable-extras=no --enable-icuio=no  --enable-layout=no \
+            --enable-tests=no --enable-samples=no || exit 1
+    ) || exit 1
+fi
+
+
 dnl ========================================================
 dnl JavaScript shell
 dnl ========================================================
 
 AC_HAVE_FUNCS(setlocale)
 AC_HAVE_FUNCS(localeconv)
 
 AC_SUBST(MOZILLA_VERSION)
@@ -4497,9 +4585,8 @@ if test "$JS_HAS_CTYPES" -a -z "$MOZ_NAT
   cache_file=$_objdir/ctypes/libffi/config.cache
   old_config_files=$CONFIG_FILES
   unset CONFIG_FILES
   AC_OUTPUT_SUBDIRS(ctypes/libffi)
   cache_file=$old_cache_file
   ac_configure_args="$_SUBDIR_CONFIG_ARGS"
   CONFIG_FILES=$old_config_files
 fi
-

--- a/js/src/jsversion.h
+++ b/js/src/jsversion.h
@@ -59,12 +59,9 @@
 
 #if USE_NEW_OBJECT_REPRESENTATION
 #  define NEW_OBJECT_REPRESENTATION_ONLY() ((void)0)
 #else
 #  define NEW_OBJECT_REPRESENTATION_ONLY() \
      MOZ_NOT_REACHED("don't call this!  to be used in the new object representation")
 #endif
 
-/* ECMAScript Internationalization API isn't fully implemented yet. */
-#define ENABLE_INTL_API 0
-
 #endif /* jsversion_h___ */