Re-apply "Merge pull request dart-lang/json_rpc_2#9 from srawlins/remove-unused-imports"

This reverts commit 16f0cbe78954615f1d3d706d79c0702fd150db58 and
re-applies commit e0459c8c981dd62425add930d329a501c5c52286.

It was determined that the build failure was caused by an upgraded SDK
version.