[dart2wasm] String.codeUnitAt on a const String is ~10x slower than equivalent Uint8List access.

[modulovalue]

String.codeUnitAt on a const _OneByteString is much slower on dart2wasm than on every other Dart backend, and ~10x slower than indexing an equivalent Uint8List on dart2wasm itself.

Benchmark: sum _LUT[a[i]] best of 5 rounds in microseconds.

	JIT	AOT	dart2js	dart2wasm
const String + codeUnitAt	50 k	43 k	73 k	786 k
hoisted Uint8List	61 k	61 k	105 k	66 k

Full repro: https://gist.github.com/modulovalue/7c4bed1762166a6e776265d0fc9abde3

• Also see: [performance] No single optimal byte-container representation across JIT, AOT, dart2js, and dart2wasm #63483