Provide source map info for simple async methods.

tests/compiler/dart2js/sourcemaps/diff.dart

 // Copyright (c) 2016, 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 sourcemap.diff;
 
 import 'package:compiler/src/io/source_file.dart';
 
 import 'html_parts.dart';
 import 'output_structure.dart';
 import 'sourcemap_helper.dart';
 import 'sourcemap_html_helper.dart';
 
-enum DiffKind {
-  UNMATCHED,
-  MATCHING,
-  IDENTICAL,
-}
+enum DiffKind { UNMATCHED, MATCHING, IDENTICAL, }
 
 /// Id for an output column.
 class DiffColumn {
   final String type;
   final int index;
 
   const DiffColumn(this.type, [this.index]);
 
   int get hashCode => type.hashCode * 19 + index.hashCode * 23;
 
@@ skipping 730 matching lines @@
         annotationType: AnnotationType.values[json['annotationType']],
         codeLocations: json['codeLocations']
             .map((j) => CodeLocation.fromJson(j, strategy))
             .toList(),
         codeSources:
             json['codeSources'].map((j) => CodeSource.fromJson(j)).toList(),
         stepInfo: json['stepInfo'],
         sourceMappingIndex: json['sourceMappingIndex']);
   }
 }