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

pkg/front_end/lib/src/fasta/dill/dill_library_builder.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_library_builder;
 
 import 'dart:convert' show JSON;
 
 import 'package:kernel/ast.dart'
     show
         Class,
         Field,
         Library,
         ListLiteral,
         Member,
         Procedure,
         StaticGet,
         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
         Builder,
         InvalidTypeBuilder,
         KernelInvalidTypeBuilder,
         KernelTypeBuilder,
         LibraryBuilder,
         Scope;
 
@@ skipping 141 matching lines @@
         isSetter = node.isSetter;
       } else if (node is Member) {
         libraryUri = node.enclosingLibrary.importUri;
         name = node.name.name;
       } else if (node is Typedef) {
         libraryUri = node.enclosingLibrary.importUri;
         name = node.name;
       } 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];
         exportScopeBuilder.addSetter(name, builder);
       } else {
         builder = library.exportScope.local[name];
         exportScopeBuilder.addMember(name, builder);
       }
       assert(node == builder.target);
     }
   }
 }