Emit warning on import of dart:mirrors.

tests/compiler/dart2js/message_kind_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/implementation/dart2jslib.dart' show
     DualKind,
     MessageKind;
 
 import 'message_kind_helper.dart';
 
 import 'dart:mirrors';
 
 main() {
   ClassMirror cls = reflectClass(MessageKind);
   Map<String, MessageKind> kinds = <String, MessageKind>{};
   cls.declarations.forEach((Symbol name, DeclarationMirror declaration) {
     if (declaration is! VariableMirror) return;
     VariableMirror variable = declaration;
     if (variable.isStatic) {
       var value = cls.getField(name).reflectee;
+      String variableName = MirrorSystem.getName(name);
+      if (variableName == 'IMPORT_EXPERIMENTAL_MIRRORS_PADDING') {
+        return;
+      }
       if (value is MessageKind) {
-        kinds[MirrorSystem.getName(name)] = value;
+        kinds[variableName] = value;
       } else {
-        Expect.fail("Weird static field: '${MirrorSystem.getName(name)}'.");
+        Expect.fail("Weird static field: '${variableName}'.");
       }
     }
   });
   List<String> names = kinds.keys.toList()..sort();
   List<String> examples = <String>[];
   for (String name in names) {
     MessageKind kind = kinds[name];
     if (name == 'GENERIC' // Shouldn't be used.
         // We can't provoke a crash.
         || name == 'COMPILER_CRASHED'
@@ skipping 10 matching lines @@
   asyncTest(() => Future.forEach(examples, (String name) {
     print("Checking '$name'.");
     Stopwatch sw = new Stopwatch()..start();
     return check(kinds[name], cachedCompiler).then((var compiler) {
       cachedCompiler = compiler;
       sw.stop();
       print("Checked '$name' in ${sw.elapsedMilliseconds}ms.");
     });
   }));
 }