Add helpers library to dart2js.

sdk/lib/_internal/compiler/implementation/helpers/helpers.dart

Index: sdk/lib/_internal/compiler/implementation/helpers/helpers.dart
diff --git a/sdk/lib/_internal/compiler/implementation/helpers/helpers.dart b/sdk/lib/_internal/compiler/implementation/helpers/helpers.dart
new file mode 100644
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+++ b/sdk/lib/_internal/compiler/implementation/helpers/helpers.dart
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+// Copyright (c) 2014, 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 for debugging helpers. The unittest analyze_unused_test checks that
+/// the helper are not used in production code.
+
+library dart2js.helpers;
+
+import "dart:collection";
+import '../dart2jslib.dart';
+import '../util/util.dart';
+
+part 'debug_collection.dart';
+part 'trace.dart';
+part 'expensive_map.dart';
+part 'expensive_set.dart';
+
+/// Print [s].
+debugPrint(s) => print(s);
+
+/// Print a message with a source location.
+reportHere(Compiler compiler, Spannable node, String debugMessage) {
+  compiler.reportInfo(node,
+      MessageKind.GENERIC, {'text': 'HERE: $debugMessage'});
+}