dart2js: Fix hints in code base.

pkg/compiler/lib/src/resolution/semantic_visitor.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.semantics_visitor;
 
 import '../constants/expressions.dart';
 import '../dart2jslib.dart' show invariant, MessageKind;
 import '../dart_types.dart';
 import '../elements/elements.dart';
-import '../helpers/helpers.dart';
 import '../tree/tree.dart';
 import '../universe/universe.dart';
 import '../util/util.dart' show Spannable, SpannableAssertionFailure;
 import 'access_semantics.dart';
 import 'operators.dart';
 import 'resolution.dart';
 import 'send_structure.dart';
 
 part 'semantic_visitor_mixins.dart';
 part 'send_resolver.dart';
@@ skipping 4598 matching lines @@
   ///       C() : this._(42);
   ///     }
   ///
   R errorUnresolvedThisConstructorInvoke(
       Send node,
       Element element,
       NodeList arguments,
       Selector selector,
       A arg);
 }