First cut at a file system abstraction for the front end.

pkg/front_end/lib/compiler_options.dart

Index: pkg/front_end/lib/compiler_options.dart
diff --git a/pkg/front_end/lib/compiler_options.dart b/pkg/front_end/lib/compiler_options.dart
index 1c496f10bc1cc88c86ba48bbe6b57a210a44f107..b5406b652fb33afb87f6e2b3b52a97c6dc4e857a 100644
--- a/pkg/front_end/lib/compiler_options.dart
+++ b/pkg/front_end/lib/compiler_options.dart
@@ -5,14 +5,12 @@
 library front_end.compiler_options;
 
 import 'compilation_error.dart';
+import 'file_system.dart';
 
 /// Callback used to report errors encountered during compilation.
 typedef void ErrorHandler(CompilationError error);
 
 /// Front-end options relevant to compiler back ends.
-///
-/// TODO(paulberry): add a mechanism to allow file system operations to be
-/// stubbed out for testing.
 class CompilerOptions {
   /// The path to the Dart SDK.
   ///
@@ -88,4 +86,15 @@ class CompilerOptions {
   /// The declared variables for use by configurable imports and constant
   /// evaluation.
   Map<String, String> declaredVariables;
+
+  /// The [FileSystem] which should be used by the front end to access files.
+  ///
+  /// TODO(paulberry): once an implementation of [FileSystem] has been created
+  /// which uses the actual physical filesystem, make that the default.
+  ///
+  /// All file system access performed by the front end goes through this
+  /// mechanism, with one exception: if no value is specified for
+  /// [packagesFilePath], the packages file is located using the actual physical
+  /// filesystem.  TODO(paulberry): fix this.
+  FileSystem fileSystem;
 }