dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/constants/constructors.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.constants.constructors;
 
 import '../dart_types.dart';
-import '../elements/elements.dart' show ConstructorElement, FieldElement;
+import '../elements/elements.dart' show FieldElement;
 import '../universe/call_structure.dart' show CallStructure;
 import '../util/util.dart';
 import 'evaluation.dart';
 import 'expressions.dart';
 
 enum ConstantConstructorKind {
   GENERATIVE,
   REDIRECTING_GENERATIVE,
   REDIRECTING_FACTORY,
 }
@@ skipping 216 matching lines @@
   }
 
   String toString() {
     StringBuffer sb = new StringBuffer();
     sb.write("{");
     sb.write("'constructor': ${targetConstructorInvocation.getText()}");
     sb.write("}");
     return sb.toString();
   }
 }