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

pkg/compiler/lib/src/universe/call_structure.dart

Index: pkg/compiler/lib/src/universe/call_structure.dart
diff --git a/pkg/compiler/lib/src/universe/call_structure.dart b/pkg/compiler/lib/src/universe/call_structure.dart
index d37d2c984be9e0d2943c709490e70c0f921cebe6..461950fdeafd3ae3524827b4d937aad6f2dc07a3 100644
--- a/pkg/compiler/lib/src/universe/call_structure.dart
+++ b/pkg/compiler/lib/src/universe/call_structure.dart
@@ -4,26 +4,14 @@
 
 library dart2js.call_structure;
 
-import 'dart:collection';
-
+import '../common.dart';
 import '../common/names.dart' show
     Identifiers,
     Names,
     Selectors;
-import '../compiler.dart' show
-    Compiler;
-import '../diagnostics/invariant.dart' show
-    invariant;
-import '../diagnostics/spannable.dart' show
-    SpannableAssertionFailure;
 import '../elements/elements.dart';
-import '../dart_types.dart';
 import '../tree/tree.dart';
-import '../types/types.dart';
 import '../util/util.dart';
-import '../world.dart' show
-    ClassWorld,
-    World;
 
 import 'selector.dart' show
     Selector;