Use FileSystem to read files in SourceLoader and TranslateUri.

pkg/front_end/lib/src/fasta/translate_uri.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.translate_uri;
 
 import 'dart:async' show Future;
 
+import 'package:front_end/file_system.dart';
 import 'package:package_config/packages_file.dart' as packages_file show parse;
 
 import 'errors.dart' show inputError;
 
-import 'io.dart' show readBytesFromFile;
-
 class TranslateUri {
   final Map<String, Uri> packages;
   final Map<String, Uri> dartLibraries;
 
   TranslateUri(this.packages, this.dartLibraries);
 
   Uri translate(Uri uri) {
     if (uri.scheme == "dart") return translateDartUri(uri);
     if (uri.scheme == "package") return translatePackageUri(uri);
     return null;
   }
 
   Uri translateDartUri(Uri uri) => dartLibraries[uri.path];
 
   Uri translatePackageUri(Uri uri) {
     int index = uri.path.indexOf("/");
     if (index == -1) return null;
     String name = uri.path.substring(0, index);
     String path = uri.path.substring(index + 1);
     Uri root = packages[name];
     if (root == null) return null;
     return root.resolve(path);
   }
 
-  static Future<TranslateUri> parse(Uri sdk, [Uri uri]) async {
+  static Future<TranslateUri> parse(FileSystem fileSystem, Uri sdk,
+      [Uri uri]) async {
     // This list below is generated with [bin/generate_dart_libraries.dart] and
     // additional entries for _builtin, _vmservice, profiler, and vmservice_io.
     //
     // TODO(ahe): This is only used with the option --compile-sdk, and
     // currently doesn't work outside the SDK source tree.
     Map<String, Uri> dartLibraries = <String, Uri>{};
     if (sdk != null) {
       dartLibraries = <String, Uri>{
         "_async_await_error_codes": sdk.resolve(
             "lib/_internal/js_runtime/lib/shared/async_await_error_codes.dart"),
@@ skipping 40 matching lines @@
         "svg": sdk.resolve("lib/svg/dartium/svg_dartium.dart"),
         "typed_data": sdk.resolve("lib/typed_data/typed_data.dart"),
         "vmservice_io": sdk.resolve("lib/vmservice_io/vmservice_io.dart"),
         "web_audio":
             sdk.resolve("lib/web_audio/dartium/web_audio_dartium.dart"),
         "web_gl": sdk.resolve("lib/web_gl/dartium/web_gl_dartium.dart"),
         "web_sql": sdk.resolve("lib/web_sql/dartium/web_sql_dartium.dart"),
       };
     }
     uri ??= Uri.base.resolve(".packages");
-    List<int> bytes =
-        await readBytesFromFile(uri, ensureZeroTermination: false);
+    List<int> bytes = await fileSystem.entityForUri(uri).readAsBytes();
     Map<String, Uri> packages = const <String, Uri>{};
     try {
       packages = packages_file.parse(bytes, uri);
     } on FormatException catch (e) {
       return inputError(uri, e.offset, e.message);
     }
     return new TranslateUri(packages, dartLibraries);
   }
 }