Replacing the rest array for MApplyArray and MConstructArray uses MApplyArgs resp. MConstructArgs. If we want to support replacing rest arrays when extra formals are present, MApplyArgs and MConstructArgs need to be updated to handle this case.

// Rest array is replaced when no extra formals are present.
function noExtraFormals(...rest) {
  return f(...rest);
}

// Rest array is _not_ replaced when extra formals are present.
function extraFormals(a, ...rest) {
  return f(...rest);
}

Pass through the number of formals, so we know how many initial arguments we have to skip
in CodeGenerator::emitPushArguments(). The actual number of arguments (argc) for
M(Apply|Construct)Args is computed through Math.max(numActuals - numFormals, 0).

https://hg.mozilla.org/mozilla-central/rev/b8defeb29902