Build up and pass MemoryFileSystem to fasta frontend.

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

 // Copyright (c) 2017, 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.
 
 /// API for compiling Dart source code to .dill (Kernel IR) files.
 library front_end.vm;
 // TODO(ahe): Convert this file to use the API in `../../kernel_generator.dart`
 // and `../../compiler_options.dart`.
 
 import 'dart:async' show Future;
 
 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.
 ///
 /// Note: The [index] property of these constants must match
 /// `Dart_KernelCompilationStatus` in
 /// [dart_api.h](../../../../runtime/include/dart_api.h).
 enum Status {
   /// Compilation was successful.
   ok,
@@ skipping 21 matching lines @@
 
   Status get status;
 
   get payload;
 
   List toResponse() => [status.index, payload];
 }
 
 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);
+      : await _findPackagesFile(fileSystem, script);
   if (packagesUri == null) {
     throw "Could not find .packages";
   }
 
   final Uri patchedSdk = Uri.base
       .resolveUri(new Uri.file(Platform.resolvedExecutable))
       .resolveUri(new Uri.directory("patched_sdk"));
 
   if (verbose) {
     print("""DFE: Requesting compilation {
   scriptUri: ${script}
   packagesUri: ${packagesUri}
   patchedSdk: ${patchedSdk}
 }""");
   }
 
   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);
   }
 }
 
 class _CompilationOk extends CompilationResult {
   final Uint8List bytes;
 
   _CompilationOk(this.bytes) : super._();
@@ skipping 40 matching lines @@
   Status get status => Status.crash;
 
   @override
   String get errorString => "${exception}\n${stack}";
 
   String toString() => "_CompilationCrash(${errorString})";
 }
 
 /// This duplicates functionality from the Loader which we can't easily
 /// access from here.
-Future<Uri> _findPackagesFile(Uri base) async {
+Future<Uri> _findPackagesFile(FileSystem fileSystem, Uri base) async {
   var dir = new File.fromUri(base).parent;
   while (true) {
     final packagesFile = dir.uri.resolve(".packages");
-    if (await new File.fromUri(packagesFile).exists()) {
+    if (await fileSystem.entityForUri(packagesFile).exists()) {
       return packagesFile;
     }
     if (dir.parent.path == dir.path) {
       break;
     }
     dir = dir.parent;
   }
   return null;
 }