[regexp] Fix writing of lastIndex in JSRegExp::Initialize.

test/test262/test262.status

 # Copyright 2011 the V8 project authors. All rights reserved.
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 # modification, are permitted provided that the following conditions are
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 #     * Redistributions of source code must retain the above copyright
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@@ skipping 447 matching lines @@
   'annexB/language/comments/single-line-html-close': [FAIL],
   'annexB/language/comments/single-line-html-close-asi': [FAIL],
   'annexB/language/comments/single-line-html-open': [FAIL],
 
   # https://bugs.chromium.org/p/v8/issues/detail?id=5137
   'annexB/built-ins/RegExp/prototype/compile/flags-undefined': [FAIL],
   'annexB/built-ins/RegExp/prototype/compile/pattern-regexp-distinct': [FAIL],
   'annexB/built-ins/RegExp/prototype/compile/pattern-regexp-same': [FAIL],
   'annexB/built-ins/RegExp/prototype/compile/pattern-undefined': [FAIL],
 
-  # https://bugs.chromium.org/p/v8/issues/detail?id=5138
-  'annexB/built-ins/RegExp/prototype/compile/pattern-regexp-immutable-lastindex': [FAIL],
-
   # https://bugs.chromium.org/p/v8/issues/detail?id=5139
   'annexB/built-ins/Date/prototype/setYear/time-clip': [FAIL],
   'annexB/built-ins/Date/prototype/setYear/year-number-relative': [FAIL],
 
   # https://bugs.chromium.org/p/v8/issues/detail?id=5140
   'annexB/built-ins/String/prototype/substr/start-to-int-err': [FAIL],
 
   ######################## NEEDS INVESTIGATION ###########################
 
   # These test failures are specific to the intl402 suite and need investigation
@@ skipping 136 matching lines @@
 }],  # asan == True
 
 ['asan == True or msan == True or tsan == True', {
   # https://bugs.chromium.org/p/v8/issues/detail?id=4639
   # The failed allocation causes an asan/msan/tsan error
   'built-ins/ArrayBuffer/allocation-limit': [SKIP],
   'built-ins/ArrayBuffer/length-is-too-large-throws': [SKIP],
 }],  # asan == True or msan == True or tsan == True
 
 ]