Provide source map info for simple async methods.

tests/compiler/dart2js/sourcemaps/lax_json_test.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.
 
 import 'package:unittest/unittest.dart';
 import 'lax_json.dart';
 
 void main() {
   test('primitives', () {
     expect(decode('true'), equals(true));
@@ skipping 33 matching lines @@
     expect(decode('["foo", "bar", true, \nnull\n,false,]'),
         equals(['foo', 'bar', true, null, false]));
   });
 
   test('map', () {
     expect(decode('{}'), equals({}));
     expect(decode('{\n}'), equals({}));
     expect(decode('{"foo":"bar"}'), equals({'foo': 'bar'}));
     expect(decode('{"foo":"bar",}'), equals({'foo': 'bar'}));
     expect(
-        decode(
-            '{"foo":true, "bar": false, "baz": true, '
-                '"boz": \nnull\n,"qux": false,}'),
+        decode('{"foo":true, "bar": false, "baz": true, '
+            '"boz": \nnull\n,"qux": false,}'),
         equals({
           'foo': true,
           'bar': false,
           'baz': true,
           'boz': null,
           'qux': false
         }));
   });
 }