Switch from IOSink to StringSink (#1105)

Towards #1100

The ExpandedReporter needs to work on the web platform. It worked before
because it only used `print` which works everywhere.

- Change the argument to `StringSink` which can be supported on every
  platform.
- Remove the `dart:io` import from the expanded reporter.
- Make the `sink` argument non-optional for the expanded reporter.
- Add a `PrintSink` implementation that buffers writes until the content
  ends with a newline and then calls `print`.