Build up and pass MemoryFileSystem to fasta frontend.

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

Index: pkg/front_end/lib/src/fasta/vm.dart
diff --git a/pkg/front_end/lib/src/fasta/vm.dart b/pkg/front_end/lib/src/fasta/vm.dart
index 353a126b0222e113619cc0d575cc1d64599f8830..c41378e9109b3fe5e03af5a4851b61ac921d1dc3 100644
--- a/pkg/front_end/lib/src/fasta/vm.dart
+++ b/pkg/front_end/lib/src/fasta/vm.dart
@@ -13,6 +13,8 @@ import 'dart:io' show File, Platform;
 
 import 'dart:typed_data' show Uint8List;
 
+import 'package:front_end/physical_file_system.dart';
+
 import 'fasta.dart' as fasta;
 
 /// Compilation status codes.
@@ -54,6 +56,13 @@ abstract class CompilationResult {
 
 Future<CompilationResult> parseScript(Uri script,
     {bool verbose: false, bool strongMode: false}) async {
+  return parseScriptInFileSystem(script, PhysicalFileSystem.instance,
+      verbose: verbose, strongMode: strongMode);
+}
+
+Future<CompilationResult> parseScriptInFileSystem(
+    Uri script, FileSystem fileSystem,
+    {bool verbose: false, bool strongMode: false}) async {
   final Uri packagesUri = (Platform.packageConfig != null)
       ? Uri.parse(Platform.packageConfig)
       : await _findPackagesFile(script);
@@ -74,7 +83,8 @@ Future<CompilationResult> parseScript(Uri script,
   }
 
   try {
-    return await fasta.parseScript(script, packagesUri, patchedSdk,
+    return await fasta.parseScriptInFileSystem(
+        script, fileSystem, packagesUri, patchedSdk,
         verbose: verbose, strongMode: strongMode);
   } catch (err, stack) {
     return new CompilationResult.crash(err, stack);