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

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.resolution.tree_elements;
 
+import '../common.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostics/invariant.dart' show
-    invariant;
-import '../diagnostics/spannable.dart' show
-    Spannable;
 import '../elements/elements.dart';
 import '../types/types.dart' show
     TypeMask;
 import '../tree/tree.dart';
 import '../util/util.dart';
-import '../universe/call_structure.dart' show
-    CallStructure;
 import '../universe/selector.dart' show
     Selector;
-import '../universe/universe.dart' show
-    UniverseSelector;
 
-import 'secret_tree_element.dart' show getTreeElement, setTreeElement;
+import 'secret_tree_element.dart' show
+    getTreeElement,
+    setTreeElement;
 import 'send_structure.dart';
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<Node> get superUses;
 
   /// Iterables of the dependencies that this [TreeElement] records of
   /// [analyzedElement].
   Iterable<Element> get allElements;
 
@@ skipping 504 matching lines @@
   }
 
   void setCurrentTypeMask(ForIn node, TypeMask mask) {
     _setTypeMask(node.inToken, mask);
   }
 
   TypeMask getCurrentTypeMask(ForIn node) {
     return _getTypeMask(node.inToken);
   }
 }