Eliminate multi-callback structure for LibraryLoader.

tests/compiler/dart2js/reexport_handled_test.dart

 // Copyright (c) 2013, 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.
 
 library reexport_handled_test;
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'mock_compiler.dart';
 import 'package:compiler/src/elements/elements.dart'
@@ skipping 12 matching lines @@
 ''';
 
 void main() {
   MockCompiler compiler;
   asyncTest(() => MockCompiler.create((MockCompiler c) {
         compiler = c;
         compiler.registerSource(exportingLibraryUri, EXPORTING_LIBRARY_SOURCE);
         compiler.registerSource(
             reexportingLibraryUri, REEXPORTING_LIBRARY_SOURCE);
         return compiler.libraryLoader.loadLibrary(exportingLibraryUri);
-      }).then((exportingLibrary) {
+      }).then((loadedLibraries) {
+        compiler.processLoadedLibraries(loadedLibraries);
+        var exportingLibrary = loadedLibraries.rootLibrary;
         Expect.isTrue(exportingLibrary.exportsHandled);
         var foo = exportingLibrary.findExported('foo');
         Expect.isNotNull(foo);
         Expect.isTrue(foo.isField);
 
         // Load reexporting library when exports are handled on the exporting library.
         return compiler.libraryLoader.loadLibrary(reexportingLibraryUri);
-      }).then((reexportingLibrary) {
-        var foo = reexportingLibrary.findExported('foo');
+      }).then((loadedLibraries) {
+        compiler.processLoadedLibraries(loadedLibraries);
+        var foo = loadedLibraries.rootLibrary.findExported('foo');
         Expect.isNotNull(foo);
         Expect.isTrue(foo.isField);
       }));
 }