[dart2wasm/spec-proposal] Experiment with "custom descriptors" / custom RTTs

[mkustermann]

Tracking bug for experimenting with WebAssembly/custom-descriptors proposal.

One place where we could take advantage of this is in closure calls: Instead of having closure instances a vtable field, we could shrink those closure objects and access the function references via the custom RTT mechanism.

Another place where we could take advantage of this is in exposing API surfaces to JS.