diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 8b54e1a5152..00c3b4dedc1 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -2958,7 +2958,7 @@ EvalInWorker(JSContext *cx, unsigned argc, jsval *vp)
     jschar *chars = (jschar *) js_malloc(str->length() * sizeof(jschar));
     if (!chars)
         return false;
-    PodCopy(chars, str->chars(), str->length());
+    CopyChars(chars, *str);
 
     WorkerInput *input = js_new<WorkerInput>(cx->runtime(), chars, str->length());
     if (!input)
@@ -4528,6 +4528,11 @@ ToLatin1(JSContext *cx, unsigned argc, Value *vp)
         return true;
     }
 
+    if (args[0].toString()->hasLatin1Chars()) {
+        args.rval().set(args[0]);
+        return true;
+    }
+
     JSLinearString *s = &args[0].toString()->asLinear();
     s->debugUnsafeConvertToLatin1();
     args.rval().setString(s);
@@ -6313,6 +6318,7 @@ main(int argc, char **argv, char **envp)
                              "to test JIT codegen (no-op on platforms other than x86 and x64).")
         || !op.addBoolOption('\0', "fuzzing-safe", "Don't expose functions that aren't safe for "
                              "fuzzers to call")
+        || !op.addBoolOption('\0', "latin1-strings", "Enable Latin1 strings (default: off)")
 #ifdef DEBUG
         || !op.addBoolOption('\0', "dump-entrained-variables", "Print variables which are "
                              "unnecessarily entrained by inner functions")
@@ -6385,6 +6391,11 @@ main(int argc, char **argv, char **envp)
 
 #endif // DEBUG
 
+    // Set this option before initializing the JSRuntime, so that Latin1 strings
+    // are used for strings allocated during initialization.
+    if (op.getBoolOption("latin1-strings"))
+        js::EnableLatin1Strings = true;
+
 #ifdef JS_THREADSAFE
     // The fake thread count must be set before initializing the Runtime,
     // which spins up the thread pool.
diff --git a/js/src/tests/lib/tests.py b/js/src/tests/lib/tests.py
index a9e35d5a1d4..ad2b96e56e2 100644
--- a/js/src/tests/lib/tests.py
+++ b/js/src/tests/lib/tests.py
@@ -14,7 +14,7 @@ TBPL_FLAGS = [
     [], # no flags, normal baseline and ion
     ['--ion-eager', '--ion-offthread-compile=off'], # implies --baseline-eager
     ['--ion-eager', '--ion-offthread-compile=off', '--ion-check-range-analysis', '--no-sse3'],
-    ['--baseline-eager'],
+    ['--baseline-eager', '--latin1-strings'],
     ['--baseline-eager', '--no-fpu'],
     ['--no-baseline', '--no-ion'],
 ]
diff --git a/js/src/vm/String.cpp b/js/src/vm/String.cpp
index df573465cb7..969a92e6d56 100644
--- a/js/src/vm/String.cpp
+++ b/js/src/vm/String.cpp
@@ -868,6 +868,8 @@ AutoStableStringChars::initTwoByte(JSContext *cx, JSString *s)
     return true;
 }
 
+bool js::EnableLatin1Strings = false;
+
 #ifdef DEBUG
 void
 JSAtom::dump()
diff --git a/js/src/vm/String.h b/js/src/vm/String.h
index ce4ead943f8..a08463d63ae 100644
--- a/js/src/vm/String.h
+++ b/js/src/vm/String.h
@@ -519,9 +519,6 @@ class JSString : public js::gc::BarrieredCell<JSString>
     void operator=(const JSString &other) MOZ_DELETE;
 };
 
-/* Temporary flag to enable Latin1 strings (bug 998392). */
-static const bool EnableLatin1Strings = false;
-
 class JSRope : public JSString
 {
     template <typename CharT>
@@ -1002,6 +999,9 @@ JS_STATIC_ASSERT(sizeof(JSAtom) == sizeof(JSString));
 
 namespace js {
 
+/* Temporary flag to enable Latin1 strings (bug 998392). */
+extern bool EnableLatin1Strings;
+
 /*
  * Thread safe RAII wrapper for inspecting the contents of JSStrings. The
  * thread safe operations such as |getCharsNonDestructive| require allocation