Add returnsNormally to async_minitest

Included change in fileapi_supported_throws_test to sanity test the
change in async_minitest.

Change-Id: I6e6b6b4eb0740ae695a85d4b3c1e2b4f8f63615a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138614
Commit-Queue: Srujan Gaddam <[email protected]>
Reviewed-by: Bob Nystrom <[email protected]>

Author: srujzs

pkg/async_helper/lib/async_minitest.dart

@@ -223,6 +223,16 @@ void _checkThrow<T>(dynamic v, void onError(error)) {
   });
 }
 
+void returnsNormally(dynamic o) {
+  try {
+    Expect.type<Function()>(o);
+    o();
+  } catch (error, trace) {
+    Expect.fail(
+        "Expected function to return normally, but threw:\n$error\n\n$trace");
+  }
+}
+
 void throws(dynamic v) {
   _checkThrow<Object>(v, (_) {});
 }

tests/lib_2/html/fileapi_supported_throws_test.dart

@@ -5,12 +5,11 @@ import 'dart:html';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:async_helper/async_minitest.dart';
-import 'package:expect/minitest.dart' as minitest;
 
 main() {
   test('requestFileSystem', () async {
-    var expectation = FileSystem.supported ? minitest.returnsNormally : throws;
-    minitest.expect(() async {
+    var expectation = FileSystem.supported ? returnsNormally : throws;
+    expect(() async {
       await window.requestFileSystem(100);
     }, expectation);
   });