Enabling WASI when building an instance sets up the default WASI
imports, and can optionally capture stdout/stderr. I'd like to set up
something similar for stdin, but Wasmer doesn't support that yet.
I also improved error handling in WasmRuntime by adding Wasmer's last
error to the thrown exceptions where appropriate.
Bug: https://github.com/dart-lang/sdk/issues/37882
Change-Id: I2d8546e878bcb43c7093490eac085aa62d318e96
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/167904
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>