pkg/compiler: fix imports

pkg/compiler/lib/src/constants/constant_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.
 
 // TODO(johnniwinther): Remove this library when all constant constructors are
 // computed during resolution.
 library dart2js.constants.constant_constructors;
 
 import '../common.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../resolution/operators.dart';
 import '../resolution/semantic_visitor.dart';
 import '../resolution/send_resolver.dart' show DeclarationResolverMixin;
 import '../resolution/send_structure.dart';
 import '../resolution/tree_elements.dart' show TreeElements;
 import '../tree/tree.dart';
 import '../universe/call_structure.dart' show CallStructure;
-
 import 'constructors.dart';
 import 'expressions.dart';
 
 ConstantConstructor computeConstantConstructor(ResolvedAst resolvedAst) {
   ConstantConstructorComputer visitor =
       new ConstantConstructorComputer(resolvedAst.elements);
   return resolvedAst.node.accept(visitor);
 }
 
 class ConstantConstructorComputer extends SemanticVisitor
@@ skipping 309 matching lines @@
   ConstantExpression visitNamedArgument(NamedArgument node) {
     return apply(node.expression);
   }
 
   @override
   ConstantExpression visitIfNull(Send node, Node left, Node right, _) {
     return new BinaryConstantExpression(
         apply(left), BinaryOperator.IF_NULL, apply(right));
   }
 }