[dart2wasm] Use Uint8List for representing bytes

We should use `Uint8List` to represent bytes in the wasm serialization
code. For cases where we only write two fixed bytes, we use `writeByte`
instead (which is faster than `Uint8List.setRange(<int>[a, b])`. We use
a cached `ByteData` to write little-endian float32/float64.

Change-Id: Id5662c1b7a3ba15f087341b13a99bbfdfa37059e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403864
Reviewed-by: Slava Egorov <vegorov@google.com>