[nullish] Stage nullish.

Bug: v8:9547
Change-Id: Ib6db47dd18b26bfc231a501723002f9e51c856ea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1849975Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64224}

src/flags/flag-definitions.h

@@ -208,7 +208,6 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
   V(harmony_regexp_sequence, "RegExp Unicode sequence properties")        \
   V(harmony_weak_refs, "harmony weak references")                         \
   V(harmony_regexp_match_indices, "harmony regexp match indices")         \
-  V(harmony_nullish, "harmony nullish operator")                          \
   V(harmony_top_level_await, "harmony top level await")
 
 #ifdef V8_INTL_SUPPORT
@@ -219,7 +218,8 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED_BASE(V)                                     \
-  V(harmony_optional_chaining, "harmony optional chaining syntax")
+  V(harmony_optional_chaining, "harmony optional chaining syntax") \
+  V(harmony_nullish, "harmony nullish operator")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_STAGED(V)                                           \