Replace ClosedWorldClassHierarchy with IncrementalClassHierarchy in SourceLoader.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index e501184da433b2a4857cdd2f0b4dcf6a3bf10206..a7fa441b63363a8e552eb9967f5f47013b6bf2b3 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -22,11 +22,13 @@ import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart'
 
 import 'package:kernel/ast.dart' show Program;
 
-import 'package:kernel/class_hierarchy.dart'
-    show ClassHierarchy, ClosedWorldClassHierarchy;
+import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
 import 'package:kernel/core_types.dart' show CoreTypes;
 
+import 'package:kernel/src/incremental_class_hierarchy.dart'
+    show IncrementalClassHierarchy;
+
 import '../builder/builder.dart'
     show
         Builder,
@@ -441,7 +443,7 @@ class SourceLoader<L> extends Loader<L> {
   }
 
   void computeHierarchy(Program program) {
-    hierarchy = new ClosedWorldClassHierarchy(program);
+    hierarchy = new IncrementalClassHierarchy();
     ticker.logMs("Computed class hierarchy");
     coreTypes = new CoreTypes(program);
     ticker.logMs("Computed core types");