Add AsyncMemoizer.future.

test/async_memoizer_test.dart

Index: test/async_memoizer_test.dart
diff --git a/test/async_memoizer_test.dart b/test/async_memoizer_test.dart
index 3289aa96b7d934b84b45ce7b00d6a62c9338a207..b8ca38a10169ffeebf1730b83487b03982992a14 100644
--- a/test/async_memoizer_test.dart
+++ b/test/async_memoizer_test.dart
@@ -21,16 +21,19 @@ main() {
   });
 
   test("forwards the return value from the function", () async {
+    expect(cache.future, completion(equals("value")));
     expect(cache.runOnce(() => "value"), completion(equals("value")));
     expect(cache.runOnce(() {}), completion(equals("value")));
   });
 
   test("forwards the return value from an async function", () async {
+    expect(cache.future, completion(equals("value")));
     expect(cache.runOnce(() async => "value"), completion(equals("value")));
     expect(cache.runOnce(() {}), completion(equals("value")));
   });
 
   test("forwards the error from an async function", () async {
+    expect(cache.future, throwsA("error"));
     expect(cache.runOnce(() async => throw "error"), throwsA("error"));
     expect(cache.runOnce(() {}), throwsA("error"));
   });