Serialize hot reload rejections in a way that can be parsed mechanically in Flutter tools

[srujzs]

Currently, we report hot reload rejections on the web in stderr during the frontend server compile:

sdk/pkg/frontend_server/lib/src/javascript_bundle.dart

Line 99 in 49ad066

throw new Exception(errors.join('/n') +

. This makes it difficult to:

1. Discern between hot reload rejections and compile-time errors. The best we can do and do is parse the string for some token e.g. Hot reload rejected.
2. Tell what hot reload rejection occurred e.g. making const classes non-const. This would be useful to report better metrics on the rejection in analytics. Today, we avoid reporting the error in analytics as user information may be mixed in.

A better solution would be to have some shared parsing functionality between the frontend server and Flutter tools like in a shared package.