CC'ing rossberg@, on-duty CF sheriff.

Please triage.

CF points to this CL

 [regexp] Named capture support for string replacements
    
    This implements support for named captures in
    RegExp.prototype[@@replace] for when the replaceValue is not callable.
    
    Named captures can be referenced from replacement strings by using the
    "$<name>" syntax. A couple of examples:
    
    let re = /(?<fst>.)(?<snd>.)/u;
    "abcd".replace(re, "$<snd>$<fst>")  // "bacd"
    "abcd".replace(re, "$2$1")     // "bacd" (numbered refs work as always)
    "abcd".replace(re, "$<snd")    // SyntaxError (unterminated named ref)
    "abcd".replace(re, "$<42$1>")  // "cd" (invalid name)
    "abcd".replace(re, "$<thd>")   // "cd" (non-existent name)
    "abcd".replace(/(?<fst>.)|(?<snd>.)/u, "$<snd>")  // "cd" (non-matched capture)
    
    Support is currently behind the --harmony-regexp-named-captures flag.
    
    BUG= v8:5437 
    
    Review-Url: https://codereview.chromium.org/2775303002
    Cr-Original-Commit-Position: refs/heads/master@{#44171}
    Committed: https://chromium.googlesource.com/v8/v8/+/17f13863b64b25eccf565e0aa9c4c441f0562b84
    Review-Url: https://codereview.chromium.org/2775303002
    Cr-Commit-Position: refs/heads/master@{#44182}

CF shows https://chromium.googlesource.com/v8/v8/+/e2858f2adc4e8767576ab3adda61e37695bfd7be as the fixing CL.