Async function returning unit triggers `assert_signature_match` assertion failure

[NathanFlurry]

Describe the Bug

When returning a unit or a value such as a Result<(), JsValue> which contains a unit from an async function, this assertion fails.

Steps to Reproduce

1. Create lib.rs containing this code:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub async fn run() {
    println!("Hello, world!");

    ()
}

2. Run rustup run nightly wasm-pack build --target web

Expected Behavior

wasm-bindgen will either convert the unit as a corresponding JsValue (likely null or undefined) or log an error at compile time.

Actual Behavior

The compilation will succeed, but wasm-bindgen will panic when the assertion fails.

Example output:

[INFO]: 🎯  Checking for the Wasm target...
[INFO]: 🌀  Compiling to Wasm...
    Finished release [optimized] target(s) in 0.08s
⚠️   [WARN]: origin crate has no README
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `0`', crates/cli-support/src/webidl/bindings.rs:208:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
Error: Running the wasm-bindgen CLI
Caused by: failed to execute `wasm-bindgen`: exited with exit code: 101

Additional Context

wasm-bindgen, wasm-bindgen-futures, js-sys, web-sys, and CLI version: master branch (93f5bba at the time of writing); also tested on 8861811 to validate this is not a regression

wasm-pack version: Both 0.9.0 or 0.8.1 do not work

Rust version: rustc 1.39.0-nightly (9b9d2aff8 2019-09-19)

[alexcrichton]

Oh dear, sorry for the runaround on the bug here! Should be fixed in #1782