Eliminate multi-callback structure for LibraryLoader.

tests/compiler/dart2js/kernel/visitor_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 IR as
 /// defined by kernel spec-mode test files.
 
 import 'dart:io';
 import 'dart:async';
 import 'package:compiler/src/compiler.dart' show Compiler;
@@ skipping 23 matching lines @@
       String name = pathlib.basenameWithoutExtension(file.path);
       bool selected = arguments.contains(name);
       if (!selected) {
         if (arguments.isNotEmpty) continue;
         if (SKIP_TESTS.contains(name)) continue;
       }
 
       test(name, () async {
         var compiler = await newCompiler();
         await compiler.run(file.absolute.uri);
-        LibraryElement library =
+        var loadedLibraries =
             await compiler.libraryLoader.loadLibrary(file.absolute.uri);
+        compiler.processLoadedLibraries(loadedLibraries);
+        var library = loadedLibraries.rootLibrary;
         JavaScriptBackend backend = compiler.backend;
         StringBuffer buffer = new StringBuffer();
         Program program = backend.kernelTask.buildProgram(library);
         new MixinFullResolution().transform(program);
         new Printer(buffer)
             .writeLibraryFile(program.mainMethod.enclosingLibrary);
         String actual = buffer.toString();
         String expected =
             new File('${TESTCASE_DIR}/spec-mode/$name.baseline.txt')
                 .readAsStringSync();
@@ skipping 16 matching lines @@
 }
 
 Future<Compiler> newCompiler() async {
   var compiler = compilerFor(
       options: [Flags.analyzeOnly, Flags.analyzeAll, Flags.useKernel]);
   await compiler.setupSdk();
 
   // The visitor no longer enqueues elements that are not reachable from the
   // program. The mixin-full resolution transform run by the test expects to
   // find dart.core::Iterator.
-  var core = await compiler.libraryLoader.loadLibrary(Uri.parse('dart:core'));
+  var loadedLibraries =
+      await compiler.libraryLoader.loadLibrary(Uri.parse('dart:core'));
+  compiler.processLoadedLibraries(loadedLibraries);
+  var core = loadedLibraries.rootLibrary;
   compiler.startResolution();
   var cls = core.implementation.localLookup('Iterator');
   cls.ensureResolved(compiler.resolution);
   return compiler;
 }