Build up and pass MemoryFileSystem to fasta frontend.

pkg/front_end/lib/src/fasta/fasta.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;
 
 import 'dart:async' show Future;
 
 import 'dart:convert' show JSON;
 
@@ skipping 146 matching lines @@
     Uri uri = c.options.output;
     await kernelTarget.writeProgram(uri,
         dumpIr: c.options.dumpIr, verify: c.options.verify);
     return uri;
   }
 }
 
 Future<CompilationResult> parseScript(
     Uri fileName, Uri packages, Uri patchedSdk,
     {bool verbose: false, bool strongMode: false}) async {
+  return parseScriptInFileSystem(
+      fileName, PhysicalFileSystem.instance, packages, patchedSdk,
+      verbose: verbose, strongMode: strongMode);
+}
+
+Future<CompilationResult> parseScriptInFileSystem(
+    Uri fileName, FileSystem fileSystem, Uri packages, Uri patchedSdk,
+    {bool verbose: false, bool strongMode: false}) async {
   try {
-    if (!await new File.fromUri(fileName).exists()) {
+    if (!await fileSystem.entityForUri(fileName).exists()) {
       return new CompilationResult.error(
           formatUnexpected(fileName, -1, "No such file."));
     }
     if (!await new Directory.fromUri(patchedSdk).exists()) {
       return new CompilationResult.error(
           formatUnexpected(patchedSdk, -1, "Patched sdk directory not found."));
     }
 
     Program program;
     try {
       TranslateUri uriTranslator =
-          await TranslateUri.parse(PhysicalFileSystem.instance, null, packages);
+          await TranslateUri.parse(fileSystem, null, packages);
       final Ticker ticker = new Ticker(isVerbose: verbose);
       final DillTarget dillTarget = new DillTarget(ticker, uriTranslator);
       _appendDillForUri(dillTarget, patchedSdk.resolve('platform.dill'));
-      final KernelTarget kernelTarget = new KernelTarget(
-          PhysicalFileSystem.instance, dillTarget, uriTranslator, strongMode);
+      final KernelTarget kernelTarget =
+          new KernelTarget(fileSystem, dillTarget, uriTranslator, strongMode);
       kernelTarget.read(fileName);
       await dillTarget.writeOutline(null);
       program = await kernelTarget.writeOutline(null);
       program = await kernelTarget.writeProgram(null);
       if (kernelTarget.errors.isNotEmpty) {
         return new CompilationResult.errors(kernelTarget.errors
             .map((err) => err.toString())
             .toList(growable: false));
       }
     } on InputError catch (e) {
@@ skipping 82 matching lines @@
   final BytesBuilder builder = new BytesBuilder();
 
   void add(List<int> data) {
     builder.add(data);
   }
 
   void close() {
     // Nothing to do.
   }
 }