In putDataProperty and putAccessorProperty we check for redundant 'puts' like this:

  if (shape->matchesParamsAfterId(obj->lastProperty()->base(), objectFlags,
                                  slot, attrs, ...)) {
    return shape;
  }

However, the BaseShape and ObjectFlags are relevant for the last property's shape and not the shape of the property we're changing.

I ran into this in bug 1700052 when I expected a test failure but the test passed because we reshaped the object due to this bug.

The object flags are only relevant for the last property's shape. Comparing them
to the flags stored in the property's shape could result in doing unnecessary work
in certain (actually redundant) cases.

https://hg.mozilla.org/mozilla-central/rev/b66357775105