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

pkg/compiler/lib/src/universe/selector.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.selector;
 
+import '../common.dart';
 import '../common/names.dart' show
     Names;
-import '../diagnostics/spannable.dart' show
-    SpannableAssertionFailure;
 import '../elements/elements.dart' show
     Element,
     Elements,
     FunctionElement,
     FunctionSignature,
     Name,
     LibraryElement,
     PublicName;
 import '../util/util.dart' show
     Hashing;
@@ skipping 253 matching lines @@
     // Add bits from the call structure.
     return Hashing.mixHashCodeBits(hash, callStructure.hashCode);
   }
 
   String toString() {
     return 'Selector($kind, $name, ${callStructure.structureToString()})';
   }
 
   Selector toCallSelector() => new Selector.callClosureFrom(this);
 }