Report internal error when an exported library isn't loaded.

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

Index: pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
index 4899ca5c9c74250fb1f4811f2960c64fbdd3ddcb..e0e5d8a7ff7f96ea052130caabfed7d9074ee830 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
@@ -18,9 +18,10 @@ import 'package:kernel/ast.dart'
         StringLiteral,
         Typedef;
 
-import '../fasta_codes.dart' show templateUnspecified;
+import '../fasta_codes.dart'
+    show templateInternalProblemExportedLibraryNotLoaded, templateUnspecified;
 
-import '../problems.dart' show unhandled, unimplemented;
+import '../problems.dart' show internalProblem, unhandled, unimplemented;
 
 import '../kernel/kernel_builder.dart'
     show
@@ -182,7 +183,14 @@ class DillLibraryBuilder extends LibraryBuilder<KernelTypeBuilder, Library> {
       } else {
         unhandled("${node.runtimeType}", "finalizeExports", -1, fileUri);
       }
-      var library = loader.read(libraryUri, -1);
+      DillLibraryBuilder library = loader.builders[libraryUri];

[scheglov, 2017/09/04 18:31:14]

What if we have mutually exporting libraries?

[ahe, 2017/09/06 07:55:50]

If they're in the same dill file, it shouldn't be a problem. I can imagine some
issues if they are in two different dill files and have external references to
each other. That's a scenario that isn't implemented/supported yet.

In fact, I think there's a lot of stuff lacking wrt to external libraries in the
dill/ folder.

+      if (library == null) {
+        internalProblem(
+            templateInternalProblemExportedLibraryNotLoaded.withArguments(
+                libraryUri, uri),
+            -1,
+            fileUri);
+      }
       Builder builder;
       if (isSetter) {
         builder = library.exportScope.setters[name];