Pass ClassHierarchy instead of creating it.

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 dc290c1cca88e804511b3d7a9ded99b02ba8927f..e501184da433b2a4857cdd2f0b4dcf6a3bf10206 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -22,7 +22,8 @@ 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;
+import 'package:kernel/class_hierarchy.dart'
+    show ClassHierarchy, ClosedWorldClassHierarchy;
 
 import 'package:kernel/core_types.dart' show CoreTypes;
 
@@ -440,7 +441,7 @@ class SourceLoader<L> extends Loader<L> {
   }
 
   void computeHierarchy(Program program) {
-    hierarchy = new ClassHierarchy(program);
+    hierarchy = new ClosedWorldClassHierarchy(program);
     ticker.logMs("Computed class hierarchy");
     coreTypes = new CoreTypes(program);
     ticker.logMs("Computed core types");