Make the `value` fields of `BoxedInt` and `BoxedDouble` `final`, to
generate immutable Wasm fields for them.
Immutable fields can potentially generate better code, as loads from the
same object will always generate the same value.
To allow making the `value` fields `final`, add a constructor.
To allow adding a constructor, "implement` base classes instead of
extending them. With `extends` the front-end wants us to call the
superclass constructors, even though they don't have any constructors.
This CL does not do the same to `BoxedBool`. We did that in a previous
iteration of the CL[1], but it caused regressions in post-TFA kernels.
[1]: https://dart-review.googlesource.com/c/sdk/+/393320
Change-Id: Ib44c25d09dcd84d999aa1d92dc9c38632454e443
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393842
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Ömer Sinan Ağacan
76717f2930