dart2js: Fix hints in code base.

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 '../dart2jslib.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../resolution/resolution.dart';
 import '../resolution/operators.dart';
 import '../resolution/semantic_visitor.dart';
 import '../resolution/send_structure.dart';
 import '../tree/tree.dart';
 import '../universe/universe.dart' show CallStructure;
 import 'expressions.dart';
-import 'values.dart';
 
 ConstantConstructor computeConstantConstructor(ResolvedAst resolvedAst) {
   ConstantConstructorComputer visitor =
       new ConstantConstructorComputer(resolvedAst.elements);
   return resolvedAst.node.accept(visitor);
 }
 
 class ConstantConstructorComputer extends SemanticVisitor
     with SemanticDeclarationResolvedMixin,
          DeclarationResolverMixin,
@@ skipping 341 matching lines @@
     }
     return new IdenticalConstantExpression(
         apply(arguments.nodes.head), apply(arguments.nodes.tail.head));
   }
 
   @override
   ConstantExpression visitNamedArgument(NamedArgument node) {
     return apply(node.expression);
   }
 }