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

pkg/compiler/lib/src/serialization/modelz.dart

Index: pkg/compiler/lib/src/serialization/modelz.dart
diff --git a/pkg/compiler/lib/src/serialization/modelz.dart b/pkg/compiler/lib/src/serialization/modelz.dart
index a4719ed3db0d0cd3d4bd11daa47ac9f4f80e2a4d..2b931729b2f2ff72a62a4f29df068bc11381af50 100644
--- a/pkg/compiler/lib/src/serialization/modelz.dart
+++ b/pkg/compiler/lib/src/serialization/modelz.dart
@@ -9,16 +9,13 @@
 
 library dart2js.serialization.modelz;
 
-import 'serialization.dart';
-import 'keys.dart';
+import '../common.dart';
 import '../common/resolution.dart' show
     Resolution;
 import '../compiler.dart'
     show Compiler;
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
-import '../diagnostics/source_span.dart'
-    show SourceSpan;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show
@@ -41,6 +38,9 @@ import '../util/util.dart' show
     Link,
     LinkBuilder;
 
+import 'keys.dart';
+import 'serialization.dart';
+
 /// Compute a [Link] from an [Iterable].
 Link toLink(Iterable iterable) {
   LinkBuilder builder = new LinkBuilder();