Use FileSystem to read files in SourceLoader and TranslateUri.

pkg/front_end/lib/src/fasta/kernel/kernel_target.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.kernel_target;
 
 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,
         AsyncMarker,
         Class,
         Constructor,
         DartType,
         DynamicType,
         EmptyStatement,
         Expression,
@@ skipping 61 matching lines @@
         MemberBuilder,
         MixinApplicationBuilder,
         NamedMixinApplicationBuilder,
         NamedTypeBuilder,
         TypeBuilder,
         TypeVariableBuilder;
 
 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;
 
   /// Shared with [CompilerContext].
   final Map<String, Source> uriToSource;
 
   SourceLoader<Library> loader;
   Program program;
 
   final List errors = [];
 
   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,
         super(dillTarget.ticker, uriTranslator) {
     resetCrashReporting();
     loader = createLoader();
   }
 
   void addError(file, int charOffset, String message) {
     Uri uri = file is String ? Uri.parse(file) : file;
     InputError error = new InputError(uri, charOffset, message);
     print(error.format());
     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) {
     String fileUri = relativizeUri(uri);
     uriToSource[fileUri] = new Source(lineStarts, sourceCode);
   }
 
   void read(Uri uri) {
     loader.read(uri);
   }
@@ skipping 587 matching lines @@
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }