Revert "Move Compiler.libraries to LibraryLoder."

tests/compiler/dart2js/deferred_follow_constant_dependencies_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.
 
 // Test that constants depended on by other constants are correctly deferred.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 import "dart:async";
@@ skipping 19 matching lines @@
                                    (name, extension) => new FakeOutputStream(),
                                    handler.diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    [],
                                    {});
   asyncTest(() => compiler.run(Uri.parse('memory:main.dart')).then((_) {
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
     var outputUnitForConstant = compiler.deferredLoadTask.outputUnitForConstant;
     var mainOutputUnit = compiler.deferredLoadTask.mainOutputUnit;
-    var lib =
-        compiler.libraryLoader.lookupLibrary(Uri.parse("memory:lib.dart"));
+    var lib = compiler.libraries["memory:lib.dart"];
     var backend = compiler.backend;
     List<Constant> allConstants = [];
 
     addConstantWithDependendencies(Constant c) {
       allConstants.add(c);
       c.getDependencies().forEach(addConstantWithDependendencies);
     }
 
     backend.constants.compiledConstants.forEach(addConstantWithDependendencies);
     for (String stringValue in ["cA", "cB", "cC"]) {
@@ skipping 24 matching lines @@
   print(lib.L);
 }
 """, "lib.dart": """
 class C {
   final a;
   const C(this.a);
 }
 
 const L = const {"cA": const C(const {"cB": "cC"})};
 """,};