Revert "Move Compiler.libraries to LibraryLoder."

tests/compiler/dart2js/deferred_load_graph_segmentation2_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.
 
 // Test of the graph segmentation algorithm used by deferred loading
 // to determine which elements can be deferred and which libraries
 // much be included in the initial download (loaded eagerly).
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 20 matching lines @@
   Compiler compiler = new Compiler(provider.readStringFromUri,
                                    (name, extension) => new FakeOutputStream(),
                                    handler.diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    [],
                                    {});
   asyncTest(() => compiler.run(Uri.parse('memory:main.dart')).then((_) {
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
     var mainOutputUnit = compiler.deferredLoadTask.mainOutputUnit;
-    var lib =
-        compiler.libraryLoader.lookupLibrary(Uri.parse("memory:lib.dart"));
+    var lib = compiler.libraries["memory:lib.dart"];
     var f1 = lib.find("f1");
     var f2 = lib.find("f2");
     Expect.notEquals(mainOutputUnit, outputUnitForElement(f1));
     Expect.equals(mainOutputUnit, outputUnitForElement(f2));
   }));
 }
 
 // The main library imports lib1 and lib2 deferred and use lib1.foo1 and
 // lib2.foo2.  This should trigger seperate outputunits for main, lib1 and lib2.
 //
@@ skipping 20 matching lines @@
 }
 """, "lib.dart": """
 int f1 () {
   return 1;
 }
 
 int f2 () {
   return 2;
 }
 """,};