Reintroduce duplicate identifier detection in preparser.

src/conversions.h

Index: src/conversions.h
diff --git a/src/conversions.h b/src/conversions.h
index 0f8d5da8ee67568a828820321e6e5a403c00c6c4..632419fab2a52ee93fb699ff3fc1413d0117c071 100644
--- a/src/conversions.h
+++ b/src/conversions.h
@@ -125,6 +125,8 @@ double StringToDouble(UnicodeCache* unicode_cache,
                       int flags,
                       double empty_string_val = 0);
 
+const int kDoubleToCStringMinBufferSize = 100;
+
 // Converts a double to a string value according to ECMA-262 9.8.1.
 // The buffer should be large enough for any floating point number.
 // 100 characters is enough.