Make source mirror ObjectMirror.getField synchronous

tests/compiler/dart2js/mirrors_metadata_test.dart

 // Copyright (c) 2012, 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 'dart:io';
 import 'dart:uri';
-import 'package:expect/expect.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/filenames.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/source_file_provider.dart';
 import 'mock_compiler.dart';
 
 const String SOURCE = 'source';
 const Uri SOURCE_URI = const Uri.fromComponents(scheme: SOURCE, path: SOURCE);
 
 MirrorSystem createMirrorSystem(String source) {
@@ skipping 58 matching lines @@
   testDeclarationComment('final int field = 0;', ['field']);
   testDeclarationComment('final field1 = 0, field2 = 0;', ['field1', 'field2']);
   testDeclarationComment('final int field1 = 0, field2 = 0;',
                          ['field1', 'field2']);
   testDeclarationComment('const field = 0;', ['field']);
   testDeclarationComment('const int field = 0;', ['field']);
   testDeclarationComment('const field1 = 0, field2 = 0;', ['field1', 'field2']);
   testDeclarationComment('const int field1 = 0, field2 = 0;',
                          ['field1', 'field2']);
 }