dart2js: Fix hints in code base.

pkg/compiler/lib/src/js/rewrite_async.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 rewrite_async;
 
 import "dart:math" show max;
 import 'dart:collection';
 
 import 'package:_internal/compiler/js_lib/shared/async_await_error_codes.dart'
     as error_codes;
 
 import "js.dart" as js;
 
 import '../util/util.dart';
 import '../dart2jslib.dart' show DiagnosticListener;
 
-import "../helpers/helpers.dart";
-
 /// Rewrites a [js.Fun] with async/sync*/async* functions and await and yield
 /// (with dart-like semantics) to an equivalent function without these.
 /// await-for is not handled and must be rewritten before. (Currently handled
 /// in ssa/builder.dart).
 ///
 /// When generating the input to this, special care must be taken that
 /// parameters to sync* functions that are mutated in the body must be boxed.
 /// (Currently handled in closure.dart).
 ///
 /// Look at [visitFun], [visitDartYield] and [visitAwait] for more explanation.
@@ skipping 2544 matching lines @@
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     hasYield = true;
     visit(node.expression);
     return true;
   }
 }