`ServiceExtensionManager` wrongly assumes service extensions are always registered on the main isolate

**Reproduction Steps:**

1. Comment out `--disable-vm-service` flag from `packages/flutter_tools/lib/src/test/runner.dart`
2. Run `flutter test --experimental-faster-testing test/material` in `flutter/packages/flutter/`
3. Copy the VM service URI into DevTools

DevTools will fail to connect to the test runner. This is due to the fact that the [service extension registration event handler](https://github.com/flutter/devtools/blob/master/packages/devtools_app_shared/lib/src/service/service_extension_manager.dart#L59) assumes the events are from the main isolate (in this example, the service extensions are registered by the Flutter framework). However, the main isolate for the test runner consists of bootstrapping code for the test and doesn't register the same service extensions, so the invocations of the service extensions fail, preventing DevTools from connecting to the application.