Eliminate multi-callback structure for LibraryLoader.

pkg/compiler/lib/src/apiimpl.dart

 // Copyright (c) 2012, 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 leg_apiimpl;
 
 import 'dart:async';
 
 import 'package:package_config/packages.dart';
 import 'package:package_config/packages_file.dart' as pkgs;
 import 'package:package_config/src/packages_impl.dart'
     show MapPackages, NonFilePackagesDirectoryPackages;
 import 'package:package_config/src/util.dart' show checkValidPackageUri;
 
 import '../compiler_new.dart' as api;
 import 'common/tasks.dart' show GenericTask, Measurer;
 import 'common.dart';
 import 'compiler.dart';
 import 'diagnostics/messages.dart' show Message;
 import 'elements/elements.dart' as elements;
 import 'environment.dart';
+import 'library_loader.dart';
 import 'io/source_file.dart';
 import 'options.dart' show CompilerOptions;
 import 'platform_configuration.dart' as platform_configuration;
 import 'resolved_uri_translator.dart';
 import 'script.dart';
 
 /// Implements the [Compiler] using a [api.CompilerInput] for supplying the
 /// sources.
 class CompilerImpl extends Compiler {
   final Measurer measurer;
@@ skipping 315 matching lines @@
     try {
       return userPackagesDiscoveryTask
           .measureIo(() => options.packagesDiscoveryProvider(uri));
     } catch (ex, s) {
       reportCrashInUserCode('Uncaught exception in package discovery', ex, s);
       rethrow;
     }
   }
 
   Uri resolvePatchUri(String libraryName) {
-    return backend.resolvePatchUri(libraryName, options.platformConfigUri);
+    return LibraryLoaderTask.resolvePatchUri(
+        libraryName, options.platformConfigUri);
   }
 }
 
 class _Environment implements Environment {
   final Map<String, String> definitions;
 
   // TODO(sigmund): break the circularity here: Compiler needs an environment to
   // initialize the library loader, but the environment here needs to know about
   // how the sdk is set up and about whether the backend supports mirrors.
   CompilerImpl compiler;
@@ skipping 41 matching lines @@
   }
 }
 
 /// For every 'dart:' library, a corresponding environment variable is set
 /// to "true". The environment variable's name is the concatenation of
 /// this prefix and the name (without the 'dart:'.
 ///
 /// For example 'dart:html' has the environment variable 'dart.library.html' set
 /// to "true".
 const String _dartLibraryEnvironmentPrefix = 'dart.library.';