Eliminate multi-callback structure for LibraryLoader.

tests/compiler/dart2js/part_of_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 part_of_test;
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/src/diagnostics/messages.dart' show MessageKind;
 import 'mock_compiler.dart';
 
 final libraryUri = Uri.parse('test:library.dart');
 const String LIBRARY_SOURCE = '''
 library foo;
 part 'part.dart';
 ''';
 
 final partUri = Uri.parse('test:part.dart');
 const String PART_SOURCE = '''
 part of bar;
 ''';
 
 void main() {
   MockCompiler compiler = new MockCompiler.internal();
   compiler.registerSource(libraryUri, LIBRARY_SOURCE);
   compiler.registerSource(partUri, PART_SOURCE);
 
-  asyncTest(() => compiler.libraryLoader.loadLibrary(libraryUri).then((_) {
-        DiagnosticCollector collector = compiler.diagnosticCollector;
-        print('errors: ${collector.errors}');
-        print('warnings: ${collector.warnings}');
-        Expect.isTrue(collector.errors.isEmpty);
-        Expect.equals(1, collector.warnings.length);
-        Expect.equals(MessageKind.LIBRARY_NAME_MISMATCH,
-            collector.warnings.first.messageKind);
-        Expect.equals(
-            'foo',
-            collector.warnings.first.message.arguments['libraryName']
-                .toString());
-      }));
+  asyncTest(
+      () => compiler.libraryLoader.loadLibrary(libraryUri).then((libraries) {
+            compiler.processLoadedLibraries(libraries);
+            DiagnosticCollector collector = compiler.diagnosticCollector;
+            print('errors: ${collector.errors}');
+            print('warnings: ${collector.warnings}');
+            Expect.isTrue(collector.errors.isEmpty);
+            Expect.equals(1, collector.warnings.length);
+            Expect.equals(MessageKind.LIBRARY_NAME_MISMATCH,
+                collector.warnings.first.messageKind);
+            Expect.equals(
+                'foo',
+                collector.warnings.first.message.arguments['libraryName']
+                    .toString());
+          }));
 }