0915f6e053
I left the mixin structure because Danny and I plan to use the same structure to start adding LSP-specific test coverage. Change-Id: I2eefc05c01eadb67f2c95fcaed3e5a963df7d69b Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339100 Reviewed-by: Konstantin Shcheglov <scheglov@google.com> Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>