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

pkg/compiler/lib/src/io/position_information.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.
 
 /// Source information system mapping that attempts a semantic mapping between
 /// offsets of JavaScript code points to offsets of Dart code points.
 
 library dart2js.source_information.position;
 
-import '../diagnostics/invariant.dart' show
-    invariant;
-import '../diagnostics/source_span.dart' show
-    SourceSpan;
-import '../diagnostics/spannable.dart' show
-    NO_LOCATION_SPANNABLE;
+import '../common.dart';
 import '../elements/elements.dart' show
     AstElement,
     LocalElement;
 import '../js/js.dart' as js;
 import '../js/js_source_mapping.dart';
 import '../js/js_debug.dart';
 import '../tree/tree.dart' show
     Node,
     Send;
 
@@ skipping 428 matching lines @@
 
   @override
   void onPositions(js.Node node,
                    int startPosition,
                    int endPosition,
                    int closingPosition) {
     codePositions.registerPositions(
         node, startPosition, endPosition, closingPosition);
   }
 }