Check ASCIIness of lhs, not this.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 7a08eec3fb5e341b070751c24e7c26eb2f9a04f8..4e20959a7a8c4b7bb6dae90feaba0e8e346865e0 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -4738,7 +4738,7 @@ bool String::SlowEquals(String* other) {
   }
 
   if (lhs->IsFlat()) {
-    if (IsAsciiRepresentation()) {
+    if (lhs->IsAsciiRepresentation()) {
       Vector<const char> vec1 = lhs->ToAsciiVector();
       if (rhs->IsFlat()) {
         if (rhs->IsAsciiRepresentation()) {