Reorganize constants/* libraries.

pkg/compiler/lib/src/serialization/element_serialization.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.serialization.elements;
 
+import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../dart2jslib.dart' show SourceSpan;
 import '../elements/elements.dart';
 import '../tree/tree.dart';
 import 'constant_serialization.dart';
 import 'keys.dart';
 import 'modelz.dart';
 import 'serialization.dart';
 
@@ skipping 460 matching lines @@
       case SerializedElementKind.TYPEVARIABLE:
         return new TypeVariableElementZ(decoder);
       case SerializedElementKind.PARAMETER:
         return new ParameterElementZ(decoder);
       case SerializedElementKind.INITIALIZING_FORMAL:
         return new InitializingFormalElementZ(decoder);
     }
     throw new UnsupportedError("Unexpected element kind '${elementKind}.");
   }
 }