Closes: #XXXX
Refs: #2785 #2399

Description

The Immutable ArrayBuffers shim can only emulate transferToImmutable on platforms with either structuredClone or ArrayBuffer.prototype.transfer. Otherwise, the shim would refuse to initialize. Without Immutable ArrayBuffers, we cannot initialize ses, and pass-style cannot implement the new byteArray Passable type, which marshal and patterns now depends on.

Unfortunately, it turns out that some platforms we want to support do not have structuredClone or ArrayBuffer.prototype.transfer. The first we encountered was Hermes, that also does not have classes with private fields. #2785 makes all these work on Hermes, by supporting sliceToImmutable instead, and using WeakMaps rather than private fields. On the theory that Hermes was a one-off, #2785 sniffed to see if it was on Hermes, only only then used the alternate shim.

In turns out Hermes was not a one-off. Some version of JSC of concern (TODO which ones?) also have neither structuredClone or ArrayBuffer.prototype.transfer. So this PR unifies the two shim strategies into one which uses the WeakMap technique, which works everywhere, and emulating sliceToImmutable, which we can do everywhere. It still sniffs for structuredClone or ArrayBuffer.prototype.transfer, and only emulates transferToImmutable if one is present. Thus, we sniff only for features not platforms, and emulate sliceToImmutable on all platforms.

Other PRs (TODO: which ones?) already ensure that pass-style, marshal, and patterns are fine with the absence of transferToImmutable.

Security Considerations

A singe piece of code with small feature detects is simpler, and thus more secure, than maintaining two alternate pieces of code, each for a different set of platforms.

Scaling Considerations

On most platforms, a class with private fields is probably more efficient than a WeakMap. This PR sacrifices that optimization in exchange for unification.

Documentation Considerations

Less to document.

Testing Considerations

The "unified" tests are just the union of the old tests, and so have two problems

• redundancy, which needs to be removed
• contains unconditional transferToImmutable tests that won't work on some platforms. These specific tests need to be made conditional, probably using the same test.skip trick some tests use for Array.prototype.transfer.

Compatibility Considerations

The goal is to make the shim compat with more platforms. On platforms that worked before this PR, this PR should be a pure refactor, and so without any compat hazards.

Upgrade Considerations

• Update NEWS.md for user-facing changes.