api tests: Have FakeHttpClient.send enqueue response handling in a new task

The Future returned by FakeHttpClient.send is created with either
Future.value or Future.error, which means it'll complete in a
*microtask*:
  https://web.archive.org/web/20170704074724/https://webdev.dartlang.org/articles/performance/event-loop#event-queue-new-future

That's too soon, as a simulation for a real API response coming over
an HTTP connection. In the live code that this simulates, the Future
completes in a new task. So, mimic that behavior.

Author: chrisbobbe

test/api/fake_api.dart

@@ -73,10 +73,10 @@ class FakeHttpClient extends http.BaseClient {
     lastRequest = request;
     switch (response) {
       case _PreparedException(:var exception):
-        return Future.error(exception);
+        return Future(() => throw exception);
       case _PreparedSuccess(:var bytes, :var httpStatus):
         final byteStream = http.ByteStream.fromBytes(bytes);
-        return Future.value(http.StreamedResponse(
+        return Future(() => http.StreamedResponse(
           byteStream, httpStatus, request: request));
     }
   }