[dart2wasm] Implement structs backed by TypedData about sdk

[mkustermann]

We currently don't consider dart:ffi as a supported thing that end users will use. There's a limited use in flutter web engine, which is supported. I believe that limited use doesn't use structs by value and is also restricted to static @Native interop usages. None of the ffi tests in the tests/ffi test suite run on wasm - we only have tests/web/wasm/ffi_native_test.dart.

Longer term we'll require multiple memories for wasm. We'll have to see what functionality of dart:ffi we want to support and how to expose it to users - maybe through dart:ffi library - maybe not.

So for now we should ensure that the ffi features which the flutter web engine requires are tested in tests/web/wasm/ffi_native_test.dart and we keep that supported.

[dcharkes]

Also, it would break nested compounds if the suggestion in #54892 is implemented, as the _offsetInBytes would not be saved anywhere.

@mkustermann @osa1 @eyebrowsoffire Are you aware of any uses of nested structs or arrays in current dart:ffi usage (only skwasm right?) in dart2wasm?

CL breaking nested compounds in dart2wasm: https://dart-review.googlesource.com/c/sdk/+/354226 (We have no dart2wasm tests exercising nested compounds. So nothing goes red.)