Fix MemoryFile.readAsLines to behave more like File.readAsLines (#147)

* Fix MemoryFile.readAsLines to behave more like File.readAsLines

Fix `MemoryFile.readAsLines` to behave more like `File.readAsLines`.
A final newline should not add an empty string as the last element of
the returned `List`.

Fixes https://github.com/google/file.dart/issues/142.

* Make RecordingFile.readAsLines/readAsLinesSync always record a final newline

Fixes https://github.com/google/file.dart/issues/146.