Make backend.setAnnotations (mostly) handle LibraryEntities.

pkg/compiler/lib/src/kernel/world_builder.dart

Index: pkg/compiler/lib/src/kernel/world_builder.dart
diff --git a/pkg/compiler/lib/src/kernel/world_builder.dart b/pkg/compiler/lib/src/kernel/world_builder.dart
index 5e9272a55585b5cb4b4b2fbf67c50c6c88cd31c0..f3548837b2fcaf79241650b8b6cad57855fd3f20 100644
--- a/pkg/compiler/lib/src/kernel/world_builder.dart
+++ b/pkg/compiler/lib/src/kernel/world_builder.dart
@@ -456,6 +456,11 @@ class KernelWorldBuilder extends KernelElementAdapterMixin {
 
   LibraryEntity getLibrary(ir.Library node) => _getLibrary(node);
 
+  ir.Library getKernelLibrary(KLibrary entity) =>
+      _libraryEnvs[entity.libraryIndex].library;
+
+  ir.Class getKernelClass(KClass entity) => _classEnvs[entity.classIndex].cls;
+
   @override
   Local getLocalFunction(ir.TreeNode node) => _getLocal(node);