Read file names from libraries.json, not URIs.

pkg/front_end/lib/src/fasta/translate_uri.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.translate_uri;
 
 import 'dart:async' show Future;
 
 import 'dart:convert' show JSON;
 
@@ skipping 80 matching lines @@
   }
 }
 
 Future<Map<String, Uri>> computeLibraries(
     FileSystem fileSystem, Uri uri) async {
   if (uri == null) return const <String, Uri>{};
   Map<String, String> libraries = JSON
       .decode(await fileSystem.entityForUri(uri).readAsString())["libraries"];
   Map<String, Uri> result = <String, Uri>{};
   libraries.forEach((String name, String path) {
-    result[name] = uri.resolve(path);
+    result[name] = uri.resolveUri(new Uri.file(path));
   });
   return result;
 }
 
 Future<Map<String, List<Uri>>> computePatches(
     FileSystem fileSystem, Uri uri) async {
   // TODO(ahe): Read patch information.
   return const <String, List<Uri>>{};
 }