Remove Uint8ListToPlainList (#143)

`Uint8List` is already a `List<int>`; we don't need a `Converter` to
go from one type to the other and can cast instead.
diff --git a/packages/file/lib/src/backends/record_replay/codecs.dart b/packages/file/lib/src/backends/record_replay/codecs.dart
index 22aacbe..c40234e 100644
--- a/packages/file/lib/src/backends/record_replay/codecs.dart
+++ b/packages/file/lib/src/backends/record_replay/codecs.dart
@@ -100,8 +100,8 @@
   void close() => _sink.close();
 }
 
-/// An Converter version of the dart:convert LineSplitter (which in
-/// 2.0 no longer implements the Converter interface)
+/// A [Converter] version of the dart:convert [LineSplitter] (which in
+/// 2.0 no longer implements the [Converter] interface).
 class LineSplitterConverter extends Converter<String, List<String>> {
   /// Creates a new [LineSplitterConverter]
   const LineSplitterConverter();
@@ -456,15 +456,6 @@
   List<T> convert(T input) => <T>[input];
 }
 
-/// Converts a [Uint8List] to a simple [List<int>].
-class Uint8ListToPlainList extends Converter<Uint8List, List<int>> {
-  /// Creates a new [Uint8ListToPlainList]
-  const Uint8ListToPlainList();
-
-  @override
-  List<int> convert(Uint8List input) => List<int>.from(input);
-}
-
 /// Revives a [Directory] entity reference into a [ReplayDirectory].
 class ReviveDirectory extends Converter<String, Directory> {
   /// Creates a new [ReviveDirectory].
diff --git a/packages/file/lib/src/backends/record_replay/replay_file.dart b/packages/file/lib/src/backends/record_replay/replay_file.dart
index 4bb850f..8eaa790 100644
--- a/packages/file/lib/src/backends/record_replay/replay_file.dart
+++ b/packages/file/lib/src/backends/record_replay/replay_file.dart
@@ -25,7 +25,7 @@
     Converter<String, Future<Uint8List>> blobToBytesFuture =
         blobToBytes.fuse(const ToFuture<Uint8List>());
     Converter<String, String> blobToString =
-        blobToBytes.fuse(const Uint8ListToPlainList()).fuse(utf8.decoder);
+        blobToBytes.cast<String, List<int>>().fuse(utf8.decoder);
     Converter<String, Future<String>> blobToStringFuture =
         blobToString.fuse(const ToFuture<String>());
     Converter<String, RandomAccessFile> reviveRandomAccessFile =