Support asynchronous tests (#600)

* Tweak the implementation of heap closures

This commit updates the implementation of the `Closure` type to internally store
an `Rc` and be suitable for dropping a `Closure` during the execution of the
closure. This is currently needed for promises but may be generally useful as
well!

* Support asynchronous tests

This commit adds support for executing tests asynchronously. This is modeled
by tests returning a `Future` instead of simply executing inline, and is
signified with `#[wasm_bindgen_test(async)]`.

Support for this is added through a new `wasm-bindgen-futures` crate which is a
binding between the `futures` crate and JS `Promise` objects.

Lots more details can be found in the details of the commit, but one of the end
results is that the `web-sys` tests are now entirely contained in the same test
suite and don't need `npm install` to be run to execute them!

* Review tweaks

* Add some bindings for `Function.call` to `js_sys`

Name them `call0`, `call1`, `call2`, ... for the number of arguments being
passed.

* Use oneshots channels with `JsFuture`

It did indeed clean up the implementation!

crates/test/sample/tests/browser.rs

@@ -0,0 +1,10 @@
+#![feature(use_extern_macros)]
+
+extern crate futures;
+extern crate sample;
+extern crate wasm_bindgen;
+extern crate wasm_bindgen_test;
+
+wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);
+
+pub mod common;

crates/test/sample/tests/common/mod.rs

@@ -0,0 +1,45 @@
+use std::time::Duration;
+
+use futures::prelude::*;
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+use sample::Timeout;
+
+#[wasm_bindgen_test]
+fn pass() {
+    console_log!("DO NOT SEE ME");
+}
+
+#[wasm_bindgen_test(async)]
+fn pass_after_2s() -> impl Future<Item = (), Error = JsValue> {
+    console_log!("immediate log");
+    Timeout::new(Duration::new(1, 0))
+        .and_then(|()| {
+            console_log!("log after 1s");
+            Timeout::new(Duration::new(1, 0)).map(|()| {
+                console_log!("log at end");
+            })
+        })
+}
+
+#[wasm_bindgen_test]
+fn fail() {
+    console_log!("helpful messsage, please see me");
+    panic!("this is a failing test");
+}
+
+#[wasm_bindgen_test(async)]
+fn fail_after_3s() -> impl Future<Item = (), Error = JsValue> {
+    console_log!("immediate log");
+    Timeout::new(Duration::new(1, 0))
+        .and_then(|()| {
+            console_log!("log after 1s");
+            Timeout::new(Duration::new(1, 0)).and_then(|()| {
+                console_log!("log after 2s");
+                Timeout::new(Duration::new(1, 0)).map(|()| {
+                    panic!("end");
+                })
+            })
+        })
+}
+

crates/test/sample/tests/node.rs

@@ -0,0 +1,8 @@
+#![feature(use_extern_macros)]
+
+extern crate futures;
+extern crate sample;
+extern crate wasm_bindgen;
+extern crate wasm_bindgen_test;
+
+pub mod common;