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');