Use common.dart to re-export commonly used entities.

pkg/compiler/lib/src/resolution/registry.dart

Index: pkg/compiler/lib/src/resolution/registry.dart
diff --git a/pkg/compiler/lib/src/resolution/registry.dart b/pkg/compiler/lib/src/resolution/registry.dart
index 51aa37c4250bc8f48dd5b956d449e5505b10d7b0..025a47d5d49d2842b890d1a3c03e55b0d8275815 100644
--- a/pkg/compiler/lib/src/resolution/registry.dart
+++ b/pkg/compiler/lib/src/resolution/registry.dart
@@ -4,6 +4,7 @@
 
 library dart2js.resolution.registry;
 
+import '../common.dart';
 import '../common/backend_api.dart' show
     Backend,
     ForeignResolver;
@@ -18,12 +19,9 @@ import '../compiler.dart' show
     Compiler;
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostics/invariant.dart' show
-    invariant;
 import '../enqueue.dart' show
     ResolutionEnqueuer;
 import '../elements/elements.dart';
-import '../helpers/helpers.dart';
 import '../tree/tree.dart';
 import '../util/util.dart' show
     Setlet;