Make the universe parts into small libraries.

pkg/compiler/lib/src/dart_backend/dart_backend.dart

Index: pkg/compiler/lib/src/dart_backend/dart_backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/dart_backend.dart b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
index 2e754f8b081f3c99aa01630ef94bf60ca1ede8cc..9c42e1d0de9c5190a451b2733f0d3c6445ae0876 100644
--- a/pkg/compiler/lib/src/dart_backend/dart_backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
@@ -52,8 +52,9 @@ import '../resolution/tree_elements.dart' show
 import '../tokens/keyword.dart' show
     Keyword;
 import '../tree/tree.dart';
+import '../universe/selector.dart' show
+    Selector;
 import '../universe/universe.dart' show
-    Selector,
     UniverseSelector;
 import '../util/util.dart';
 import 'backend_ast_to_frontend_ast.dart' as backend2frontend;