Flake in TypeConversionsTest.CanConvertListOfStrings

Seen on: https://github.com/flutter/engine/pull/25706

Backtrace:

```
[ RUN      ] TypeConversionsTest.CanConvertEmptyList
[       OK ] TypeConversionsTest.CanConvertEmptyList (53 ms)
[ RUN      ] TypeConversionsTest.CanConvertListOfStrings
[FATAL:flutter/runtime/dart_isolate.cc(358)] Check failed: GetMessageHandlingTaskRunner()->RunsTasksOnCurrentThread(). 
```