Eliminate multi-callback structure for LibraryLoader.

tests/compiler/dart2js/kernel/class_hierarchy_test.dart

Index: tests/compiler/dart2js/kernel/class_hierarchy_test.dart
diff --git a/tests/compiler/dart2js/kernel/class_hierarchy_test.dart b/tests/compiler/dart2js/kernel/class_hierarchy_test.dart
index bb90c675101a001c440b33f342a6b0c7ecb04161..bb18502397497e7ac5861d3ed4c59b2504b0f87e 100644
--- a/tests/compiler/dart2js/kernel/class_hierarchy_test.dart
+++ b/tests/compiler/dart2js/kernel/class_hierarchy_test.dart
@@ -5,10 +5,11 @@
 /// Test that the dart2js copy of [KernelVisitor] generates the expected class
 /// hierarchy.
 
+import 'package:compiler/src/commandline_options.dart' show Flags;
 import 'package:compiler/src/compiler.dart' show Compiler;
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/js_backend/backend.dart' show JavaScriptBackend;
-import 'package:compiler/src/commandline_options.dart' show Flags;
+import 'package:compiler/src/library_loader.dart' show LoadedLibraries;
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/class_hierarchy.dart';
 import 'package:test/test.dart';
@@ -37,9 +38,11 @@ main(List<String> arguments) {
   test('mixin', () async {
     Uri mainUri = Uri.parse('memory:main.dart');
     await compiler.run(mainUri);
-    LibraryElement library = await compiler.libraryLoader.loadLibrary(mainUri);
+    LoadedLibraries libraries =
+        await compiler.libraryLoader.loadLibrary(mainUri);
+    compiler.processLoadedLibraries(libraries);
     JavaScriptBackend backend = compiler.backend;
-    ir.Program program = backend.kernelTask.buildProgram(library);
+    ir.Program program = backend.kernelTask.buildProgram(libraries.rootLibrary);
     ClassHierarchy hierarchy = new ClassHierarchy(program);
 
     ir.Class getClass(String name) {