Include uriToSource from dill-based libraries in output

pkg/front_end/lib/src/fasta/dill/dill_loader.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.dill_loader;
 
 import 'dart:async' show Future;
 
-import 'package:kernel/ast.dart' show Library, Program;
+import 'package:kernel/ast.dart' show Library, Program, Source;
 
 import '../loader.dart' show Loader;
 import '../target_implementation.dart' show TargetImplementation;
 import 'dill_library_builder.dart' show DillLibraryBuilder;
 
 class DillLoader extends Loader<Library> {
   /// Source targets are compiled against these binary libraries.
   final libraries = <Library>[];
 
+  /// Sources for all appended programs.

[scheglov, 2017/06/09 15:32:56]

"appended libraries"

+  final Map<String, Source> uriToSource = <String, Source>{};
+
   DillLoader(TargetImplementation target) : super(target);
 
   /// Append compiled libraries from the given [program]. If the [filter] is
   /// provided, append only libraries whose [Uri] is accepted by the [filter].
   List<DillLibraryBuilder> appendLibraries(Program program,
       [bool filter(Uri uri)]) {
     var builders = <DillLibraryBuilder>[];
     for (Library library in program.libraries) {
       if (filter == null || filter(library.importUri)) {
         libraries.add(library);
         DillLibraryBuilder builder = read(library.importUri, -1);
         builder.library = library;
         builders.add(builder);
       }
     }
+    uriToSource.addAll(program.uriToSource);

[scheglov, 2017/06/09 15:32:56]

We're adding only some libraries, why do we add ALL sources?

[Johnni Winther, 2017/06/09 15:46:23]

To ensure soundness. It would of course be an optimization to find only the
needed sources.

     return builders;
   }
 
   Future<Null> buildOutline(DillLibraryBuilder builder) async {
     builder.library.classes.forEach(builder.addClass);
     builder.library.procedures.forEach(builder.addMember);
     builder.library.typedefs.forEach(builder.addTypedef);
     builder.library.fields.forEach(builder.addMember);
   }
 
   Future<Null> buildBody(DillLibraryBuilder builder) {
     return buildOutline(builder);
   }
 }