Enable slices of external strings (in the tentative implementation).

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index d1fe1c7c1675377ead4e8d7bdc8d7717630abe07..4ddee7c38e5390a9e3970b45fbc996606b5816e7 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -6029,7 +6029,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_StringSplit) {
       }
     } else {
       Vector<const uc16> subject_vector = subject_content.ToUC16Vector();
-      if (pattern->IsAsciiRepresentation()) {
+      if (pattern_content.IsAscii()) {
         FindStringIndices(isolate,
                           subject_vector,
                           pattern_content.ToAsciiVector(),