Hide synthetic metadata field exposed in r35926.

tests/lib/mirrors/regress_19731_test.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.
+
+@metadata
+library regress_19731;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+@metadata
+const metadata = const Object();
+
+class OneField {
+  @metadata
+  var onlyClassField;
+
+  @metadata
+  method() {}
+}
+
+@metadata
+method() {}
+
+main() {
+  var classFieldNames = reflectType(OneField).declarations.values
+    .where((v) => v is VariableMirror).map((v) => v.simpleName).toList();
+  Expect.setEquals([#onlyClassField], classFieldNames);
+
+  var libraryFieldNames = reflectType(OneField).owner.declarations.values
+    .where((v) => v is VariableMirror).map((v) => v.simpleName).toList();
+  Expect.setEquals([#metadata], libraryFieldNames);
+}