Use FileSystem to read files in SourceLoader and TranslateUri.

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

Index: pkg/front_end/lib/src/fasta/compiler_context.dart
diff --git a/pkg/front_end/lib/src/fasta/compiler_context.dart b/pkg/front_end/lib/src/fasta/compiler_context.dart
index 040858c81ca4181fce81dd958968a743c8e42dfe..3994f09381d18f905e5c222b938a69a642e6fa99 100644
--- a/pkg/front_end/lib/src/fasta/compiler_context.dart
+++ b/pkg/front_end/lib/src/fasta/compiler_context.dart
@@ -6,6 +6,8 @@ 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;
@@ -18,6 +20,7 @@ final CompilerContext rootContext =
     new CompilerContext(CompilerCommandLine.forRootContext());
 
 class CompilerContext {
+  final FileSystem fileSystem = PhysicalFileSystem.instance;
   final CompilerCommandLine options;
 
   final Map<String, Source> uriToSource = <String, Source>{};