assert: make t.throws() accept async function as parameter.

Refs: #1371

Author: oantoro

lib/assert.js

@@ -182,13 +182,14 @@ function wrapAssertions(callbacks) {
 coreAssertThrowsErrorArg = err;
 }
 
+let maybePromise;
 const test = (fn, stack) => {
 let actual;
 let threw = false;
 try {
 coreAssert.throws(() => {
 try {
-fn();
+maybePromise = fn();
 } catch (err) {
 actual = err;
 threw = true;
@@ -208,7 +209,7 @@ function wrapAssertions(callbacks) {
 }
 };
 
-if (promise) {
+const handlePromise = promise => {
 // Record stack before it gets lost in the promise chain.
 const stack = getStack();
 const intermediate = promise.then(value => {
@@ -222,13 +223,25 @@ function wrapAssertions(callbacks) {
 pending(this, intermediate);
 // Don't reject the returned promise, even if the assertion fails.
 return intermediate.catch(noop);
+};
+
+if (promise) {
+return handlePromise.call(this, promise);
 }
 
 try {
 const retval = test(fn);
 pass(this);
 return retval;
 } catch (err) {
+if (maybePromise) {
+if (isPromise(maybePromise)) {
+return handlePromise.call(this, maybePromise);
+}
+if (isObservable(maybePromise)) {
+return handlePromise.call(this, observableToPromise(maybePromise));
+}
+}
 fail(this, err);
 }
 },

test/assert.js

@@ -678,6 +678,17 @@ test('.throws() returns the rejection reason of promise', t => {
 });
 });
 
+test('.throws() returns the rejection reason of function that return rejected `Promise`', t => {
+const expected = new Error();
+
+return assertions.throws(() => {
+return Promise.reject(expected);
+}).then(actual => {
+t.is(actual, expected);
+t.end();
+});
+});
+
 test('.throws() fails if passed a bad value', t => {
 failsWith(t, () => {
 assertions.throws('not a function');