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

pkg/compiler/lib/src/elements/elements.dart

 // Copyright (c) 2012, 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 elements;
 
+import '../common.dart';
 import '../common/resolution.dart' show
     Resolution;
 import '../compiler.dart' show
     Compiler;
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
-import '../diagnostics/diagnostic_listener.dart';
-import '../diagnostics/messages.dart' show
-MessageKind;
-import '../diagnostics/source_span.dart' show
-    SourceSpan;
-import '../diagnostics/spannable.dart' show
-    Spannable;
 import '../resolution/scope.dart' show
     Scope;
 import '../resolution/tree_elements.dart' show
     TreeElements;
 import '../ordered_typeset.dart' show OrderedTypeSet;
 import '../script.dart';
 import '../tokens/token.dart' show
     Token,
     isUserDefinableOperator,
     isMinusOperator;
@@ skipping 1677 matching lines @@
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }