Remove deprecated_internalProblem.

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 6da95772c17e1fa988b9dc7635dd1f5a5ef0a91b..0ce8967160a4779b15c4a0af7e47500875572194 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
@@ -8,9 +8,12 @@ import 'dart:async' show Future;
 
 import 'package:kernel/ast.dart' show Library, Program, Source;
 
-import '../deprecated_problems.dart' show deprecated_internalProblem;
 import '../loader.dart' show Loader;
+
+import '../problems.dart' show unhandled;
+
 import '../target_implementation.dart' show TargetImplementation;
+
 import 'dill_library_builder.dart' show DillLibraryBuilder;
 
 class DillLoader extends Loader<Library> {
@@ -41,8 +44,7 @@ class DillLoader extends Loader<Library> {
 
   Future<Null> buildOutline(DillLibraryBuilder builder) async {
     if (builder.library == null) {
-      deprecated_internalProblem(
-          "Builder.library for ${builder.uri} should not be null.");
+      unhandled("null", "builder.library", 0, builder.fileUri);
     }
     builder.library.classes.forEach(builder.addClass);
     builder.library.procedures.forEach(builder.addMember);