prepare wip

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

test/api/fake_api.dart

@@ -62,6 +62,8 @@ class FakeHttpClient extends http.BaseClient {
     Duration delay = Duration.zero,
     bool append = false,
   }) {
+    // TODO: ensure that there is no outstanding prepared responses during
+    // tear down.
     assert(append || _preparedResponses.isEmpty,
       'FakeApiConnection.prepare was called while already expecting a request');
     if (exception != null) {