Remove dependency on the Path class from dart:io
Remove dependency on the Path class from dart:io

The Path class was removed from dart:io in Dart r26191. With this
change URIs are used to manipulate paths instead.

R=ajohnsen@google.com
BUG=

Review URL: https://chromiumcodereview.appspot.com//23249004

dart-lang/dart-protobuf,taisph/dart-protoc-plugin,taisph/dart-protoc-plugin,taisph/dart-protoc-plugin,dart-lang/dart-protobuf,dart-lang/protobuf,google/protobuf.dart,dart-lang/dart-protobuf,bwu-dart-contributing/dart-protoc-plugin,dart-lang/protobuf,google/protobuf.dart,google/protobuf.dart,dart-lang/protobuf