Ship the relative indexing method `.at`

The bricklink.com brickage has since been fixed upstream.

I2S with LGTMs:
https://groups.google.com/a/chromium.org/g/blink-dev/c/I8S78w7aFmE/m/qLHAcjhRCQAJ

Bug: v8:10961
Bug: chromium:1170196
Change-Id: I460e1080aee837bde0e9861d761f2e7dbc8fd6b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2824101
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73962}

src/flags/flag-definitions.h

@@ -286,7 +286,6 @@ DEFINE_BOOL(harmony_shipping, true, "enable all shipped harmony features")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED_BASE(V)                                              \
-  V(harmony_relative_indexing_methods, "harmony relative indexing methods") \
   V(harmony_class_static_blocks, "harmony static initializer blocks")       \
   V(harmony_error_cause, "harmony error cause property")
 
@@ -302,7 +301,8 @@ DEFINE_BOOL(harmony_shipping, true, "enable all shipped harmony features")
   V(harmony_atomics, "harmony atomics")                           \
   V(harmony_regexp_match_indices, "harmony regexp match indices") \
   V(harmony_private_brand_checks, "harmony private brand checks") \
-  V(harmony_top_level_await, "harmony top level await")
+  V(harmony_top_level_await, "harmony top level await")           \
+  V(harmony_relative_indexing_methods, "harmony relative indexing methods")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_SHIPPING(V)             \

test/test262/test262.status

@@ -573,11 +573,6 @@
   # http://crbug/v8/10905
   'language/identifier-resolution/assign-to-global-undefined': [FAIL],
 
-  # https://bugs.chromium.org/p/chromium/issues/detail?id=1159764
-  'built-ins/Array/prototype/at/*': [FAIL],
-  'built-ins/String/prototype/at/*': [FAIL],
-  'built-ins/TypedArray/prototype/at/*': [FAIL],
-
   # http://crbug/v8/11530
   'built-ins/Function/internals/Call/class-ctor-realm': [FAIL],