Eliminate multi-callback structure for LibraryLoader.

tests/compiler/dart2js/library_resolution_test.dart

 // Copyright (c) 2014, 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.
 
 /// Check that relative URIs are resolved against the canonical name of a
 /// library. This only matters for dart:-libraries, so this test mocks up two
 /// dart:-libraries.
 
 import "dart:io";
 
 import "dart:async";
 
 import "memory_source_file_helper.dart";
 
 import "package:async_helper/async_helper.dart";
 
 import 'package:expect/expect.dart' show Expect;
 
 import 'package:compiler/src/diagnostics/messages.dart'
     show MessageKind, MessageTemplate;
 
 import 'package:compiler/src/elements/elements.dart' show LibraryElement;
 
+import 'package:compiler/src/library_loader.dart' show LoadedLibraries;
+
 import 'package:compiler/src/null_compiler_output.dart' show NullCompilerOutput;
 
 import 'package:compiler/src/old_to_new_api.dart'
     show LegacyCompilerDiagnostics, LegacyCompilerInput;
 import 'package:compiler/src/options.dart' show CompilerOptions;
 
 Uri sdkRoot = Uri.base.resolve("sdk/");
 Uri mock1LibraryUri = sdkRoot.resolve("lib/mock1.dart");
 Uri mock2LibraryUri = sdkRoot.resolve("lib/mock2.dart");
 
@@ skipping 30 matching lines @@
   int actualMessageCount = 0;
 
   wrappedHandler(Uri uri, int begin, int end, String message, kind) {
     if (message == expectedMessage) {
       actualMessageCount++;
     } else {
       return handler(uri, begin, end, message, kind);
     }
   }
 
-  checkLibrary(LibraryElement library) {
+  checkLibrary(LoadedLibraries libraries) {

[Siggi Cherem (dart-lang), 2017/03/24 17:40:28]

checkLibrary => checkLibraries?

[Emily Fortuna, 2017/03/24 18:30:20]

Done.

     Expect.equals(1, actualMessageCount);
   }
 
   CompilerImpl compiler = new CustomCompiler(
       new LegacyCompilerInput(wrappedProvider),
       new LegacyCompilerDiagnostics(wrappedHandler),
       sdkRoot,
       packageConfig);
 
   asyncStart();
   await compiler.setupSdk();
   // TODO(het): Find cleaner way to do this
   compiler.resolvedUriTranslator.sdkLibraries['m_o_c_k_1'] = mock1LibraryUri;
   compiler.resolvedUriTranslator.sdkLibraries['m_o_c_k_2'] = mock2LibraryUri;
-  var library =
+  var libraries =
       await compiler.libraryLoader.loadLibrary(Uri.parse("dart:m_o_c_k_1"));
-  await checkLibrary(library);
+  await checkLibrary(libraries);
   asyncSuccess(null);
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   "mock1.dart": "library mock1; import 'mock2.dart';",
   "mock2.dart": "library mock2;",
 };