Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/helpers/helpers.dart

 // 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:async' show EventSink;
@@ skipping 49 matching lines @@
 /// Wraps the call to [f] with a print of 'start:$s' and 'end:$s' incrementing
 /// the current indentation used by [debugPrint] during the execution of [f].
 ///
 /// Use this to get a tree-like debug printout for nested calls.
 DebugWrapPrint get debugWrapPrint {
   enableDebugMode();
   return _debugWrapPrint;
 }
 
 /// Implementation of [debugWrapPrint].
-DebugWrapPrint _debugWrapPrint(s, f()) {
+_debugWrapPrint(s, f()) {
   debugPrint('start:$s');
   var result = _indentation.indentBlock(f);
   debugPrint('end:$s');
   return result;
 }
 
 /// Dummy method to mark breakpoints.
 debugBreak() {
   enableDebugMode();
 }
 
 /// Function signature of [reportHere].
 typedef ReportHere(Compiler compiler, Spannable node, String debugMessage);
 
 /// Print a message with a source location.
 ReportHere get reportHere {
   enableDebugMode();
   return _reportHere;
 }
 
 /// Implementation of [reportHere]
 _reportHere(Compiler compiler, Spannable node, String debugMessage) {
   compiler.reportInfo(node,
       MessageKind.GENERIC, {'text': 'HERE: $debugMessage'});
 }