Allow non-void promises in assertThrowsAsync

std/testing/asserts.ts

@@ -325,8 +325,8 @@ export function fail(msg?: string): void {
  * throws.  An error class and a string that should be included in the
  * error message can also be asserted.
  */
-export function assertThrows(
-  fn: () => void,
+export function assertThrows<T = void>(
+  fn: () => T,
   ErrorClass?: Constructor,
   msgIncludes = "",
   msg?: string
@@ -361,8 +361,8 @@ export function assertThrows(
   return error;
 }
 
-export async function assertThrowsAsync(
-  fn: () => Promise<void>,
+export async function assertThrowsAsync<T = void>(
+  fn: () => Promise<T>,
   ErrorClass?: Constructor,
   msgIncludes = "",
   msg?: string

std/testing/asserts_test.ts

@@ -9,6 +9,7 @@ import {
   assertEquals,
   assertStrictEq,
   assertThrows,
+  assertThrowsAsync,
   AssertionError,
   equal,
   fail,
@@ -245,6 +246,20 @@ test("testingAssertFailWithWrongErrorClass", function (): void {
   );
 });
 
+test("testingAssertThrowsWithReturnType", () => {
+  assertThrows(() => {
+    throw new Error();
+    return "a string";
+  });
+});
+
+test("testingAssertThrowsAsyncWithReturnType", () => {
+  assertThrowsAsync(() => {
+    throw new Error();
+    return Promise.resolve("a Promise<string>");
+  });
+});
+
 const createHeader = (): string[] => [
   "",
   "",