Revert "Move Compiler.libraries to LibraryLoder."

tests/compiler/dart2js/tag_mapping_test.dart

Index: tests/compiler/dart2js/tag_mapping_test.dart
diff --git a/tests/compiler/dart2js/tag_mapping_test.dart b/tests/compiler/dart2js/tag_mapping_test.dart
index 04af1fcef6bb65b07fab895a1d334f13e1b2c3bf..3ae88f4f20efa77462188884b00283e1b208bae4 100644
--- a/tests/compiler/dart2js/tag_mapping_test.dart
+++ b/tests/compiler/dart2js/tag_mapping_test.dart
@@ -26,10 +26,8 @@ void main() {
   };
 
   asyncTest(() => compileSources(sources, (MockCompiler compiler) {
-    LibraryElement mainApp =
-        compiler.libraryLoader.lookupLibrary(Uri.parse('source:/main.dart'));
-    LibraryElement lib =
-        compiler.libraryLoader.lookupLibrary(Uri.parse('source:/library.dart'));
+    LibraryElement mainApp = compiler.libraries['source:/main.dart'];
+    LibraryElement lib = compiler.libraries['source:/library.dart'];
     Expect.isNotNull(mainApp, 'Could not find main.dart library');
     Expect.isNotNull(lib, 'Could not find library.dart library');