fix #29530, field metadata generation

tests/lib_strong/mirrors/field_metadata2_test.dart

+// compile options: --emit-metadata
+// Copyright (c) 2017, 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.
+
+// Run essentially the same test, but with emit-metadata compile option,
+// which allows us to reflect on the fields.
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+import 'field_metadata_test.dart' as field_metadata_test;
+import 'field_metadata_test.dart' show Foo, Bar;
+
+void main() {
+  // Make sure the other test still works.
+  field_metadata_test.main();
+
+  // Check that we can now reflect on the annotations.
+  dynamic f = new Foo();
+  var members = reflect(f).type.declarations;
+  var bar = members[#x].metadata.first.reflectee as Bar;
+  Expect.equals(bar.name, 'bar');
+
+  var baz = members[#y].metadata.first.reflectee as Bar;
+  Expect.equals(baz.name, 'baz');
+}