Eliminate multi-callback structure for LibraryLoader.

tests/compiler/dart2js/kernel/class_hierarchy_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// 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';
 
 import '../memory_compiler.dart';
 
 main(List<String> arguments) {
   Compiler compiler = compilerFor(memorySourceFiles: {
     'main.dart': '''
       class S {
         sMethod() {}
       }
       class M {
         mMethod() {}
       }
       class C extends S with M {
         cMethod() {}
       }
       main() {}
       '''
   }, options: [
     Flags.analyzeOnly,
     Flags.analyzeAll,
     Flags.useKernel
   ]);
   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) {
       for (ir.Class cls in hierarchy.classes) {
         if (cls.enclosingLibrary.importUri == mainUri && cls.name == name) {
           if (arguments.contains('-v')) {
             print('$cls');
             print(' dispatch targets:');
             hierarchy
                 .getDispatchTargets(cls)
@@ skipping 15 matching lines @@
             hierarchy.getDispatchTarget(subClass, member.name), equals(member),
             reason: 'Unexpected dispatch target for ${member.name} '
                 'in $subClass');
       }
     }
 
     checkInheritance(classS, classC);
     checkInheritance(classM, classC);
   });
 }