Include uriToSource from dill-based libraries in output

pkg/front_end/lib/src/fasta/dill/dill_loader.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_loader.dart b/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
index b8537da9d2caddddad36a36cc7d434ac0bbf364a..7279ca74b2457c827053af5683c71bf94bf18057 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
@@ -6,7 +6,7 @@ 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;
@@ -16,6 +16,9 @@ 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
@@ -31,6 +34,7 @@ class DillLoader extends Loader<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;
   }