The latest version of `test_api` removed the deprecated annotation for a couple libraries. The deprecations had been a hack to avoid usage from inappropriate places, but they cause trouble and likely weren't effective for that goal.

Remove separate import for `registerException` since that was also moved to be available from the top level `scaffolding.dart` library.

* opt out the flutter tool

* oops EOF

* fix import

* Update tool_backend.dart

* Update daemon_client.dart

* fix more

Use the new single `src/platform.dart` import to pick up what is needed
to implement a test platform from both `test_api` and `test_core`.