Use FileSystem to read files in SourceLoader and TranslateUri.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index 0879208eac7ac8610a647ed7038c6f6aa621c921..ea33e1870e7d5cd1093366f04763218528bcca82 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -8,6 +8,7 @@ import 'dart:async' show Future;
 
 import 'dart:io' show File, IOSink;
 
+import 'package:front_end/file_system.dart';
 import 'package:kernel/ast.dart'
     show
         Arguments,
@@ -89,6 +90,9 @@ import 'kernel_builder.dart'
 import 'verifier.dart' show verifyProgram;
 
 class KernelTarget extends TargetImplementation {
+  /// The [FileSystem] which should be used to access files.
+  final FileSystem fileSystem;
+
   final bool strongMode;
 
   final DillTarget dillTarget;
@@ -104,8 +108,8 @@ class KernelTarget extends TargetImplementation {
   final TypeBuilder dynamicType =
       new KernelNamedTypeBuilder("dynamic", null, -1, null);
 
-  KernelTarget(
-      DillTarget dillTarget, TranslateUri uriTranslator, this.strongMode,
+  KernelTarget(this.fileSystem, DillTarget dillTarget,
+      TranslateUri uriTranslator, this.strongMode,
       [Map<String, Source> uriToSource])
       : dillTarget = dillTarget,
         uriToSource = uriToSource ?? CompilerContext.current.uriToSource,
@@ -121,7 +125,8 @@ class KernelTarget extends TargetImplementation {
     errors.add(error);
   }
 
-  SourceLoader<Library> createLoader() => new SourceLoader<Library>(this);
+  SourceLoader<Library> createLoader() =>
+      new SourceLoader<Library>(fileSystem, this);
 
   void addSourceInformation(
       Uri uri, List<int> lineStarts, List<int> sourceCode) {