Revert "[vm, reload] Delay enum forwarding until after instance morphing."

This reverts commit 6ef967fb07734cc895bf535ce09165e2d49eb892.

Reason for revert: https://github.com/dart-lang/sdk/issues/53039

Original change's description:
> [vm, reload] Delay enum forwarding until after instance morphing.
>
> Enum forwarding requires evaluating and canonicalizing the new enum instances. If these in turn refer to other instances of classes with shape changes, canonicalization may try to compare instances of the old and new shapes and dereference past the end of an object. By waiting for instance morphing to complete, canonicalization can only see instances with the new shape.
>
> Also, don't attempt to forward Enum.values. When an enum member is added or removed, this has the same merging problem as 40442.
>
> Cf. bad074cc4921e33a5142646c265b34b55c04d145.
>
> TEST=ci
> Bug: https://github.com/flutter/flutter/issues/129177
> Change-Id: I19d0508059bade8496000eeea257bb0730f11d17
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/316041
> Reviewed-by: Ben Konyi <bkonyi@google.com>
> Commit-Queue: Ryan Macnak <rmacnak@google.com>

Bug: https://github.com/flutter/flutter/issues/129177
Change-Id: I8aa09be5d8fd72460ab26294ed94a5075135d48b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/316501
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>