Start using String type info:

src/type-info.cc

Index: src/type-info.cc
diff --git a/src/type-info.cc b/src/type-info.cc
index b1bde599f4c6c1684a993c2c054595845d53d492..3fc929db3ca0c999e4851c9dbeee8fca6d83af76 100644
--- a/src/type-info.cc
+++ b/src/type-info.cc
@@ -41,6 +41,8 @@ TypeInfo TypeInfo::TypeFromValue(Handle<Object> value) {
     info = TypeInfo::IsInt32Double(HeapNumber::cast(*value)->value())
         ? TypeInfo::Integer32()
         : TypeInfo::Double();
+  } else if (value->IsString()) {
+    info = TypeInfo::String();
   } else {
     info = TypeInfo::Unknown();
   }