Use FileSystem to read files in SourceLoader and TranslateUri.

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

 // Copyright (c) 2017, 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.compiler_context;
 
 import 'dart:async' show Zone, runZoned;
 
+import 'package:front_end/file_system.dart';
+import 'package:front_end/physical_file_system.dart';
 import 'package:kernel/ast.dart' show Source;
 
 import 'compiler_command_line.dart' show CompilerCommandLine;
 
 import 'colors.dart' show computeEnableColors;
 
 final Object compilerContextKey = new Object();
 
 final CompilerContext rootContext =
     new CompilerContext(CompilerCommandLine.forRootContext());
 
 class CompilerContext {
+  final FileSystem fileSystem = PhysicalFileSystem.instance;
   final CompilerCommandLine options;
 
   final Map<String, Source> uriToSource = <String, Source>{};
 
   bool enableColorsCached = null;
 
   CompilerContext(this.options);
 
   static CompilerContext get current {
     return Zone.current[compilerContextKey] ?? rootContext;
   }
 
   /// Perform [action] in a [Zone] where [cl] will be available as
   /// `CompilerContext.current.options`.
   static dynamic withGlobalOptions(
       CompilerCommandLine cl, dynamic action(CompilerContext c)) {
     CompilerContext c = new CompilerContext(cl);
     return runZoned(() => action(c), zoneValues: {compilerContextKey: c});
   }
 
   static bool get enableColors {
     return current.enableColorsCached ??= computeEnableColors(current);
   }
 }