Tighten up handling of JS types and add test.

tests/lib_strong/html/js_test.dart

 // Copyright (c) 2013, 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 'dart:html';
 import 'dart:typed_data' show Int32List;
 import 'dart:indexed_db' show IdbFactory, KeyRange;
 import 'dart:js';
 import 'package:js/js_util.dart' as js_util;
 
@@ skipping 690 matching lines @@
       final set = new Set.from([1, 2, 3, 4, 5, 6, 7, 8]);
       var array = new JsObject.jsify(set);
       expect(context.callMethod('isArray', [array]), isTrue);
       expect(array['length'], equals(set.length));
       for (var i = 0; i < array['length']; i++) {
         expect(set.contains(array[i]), isTrue);
       }
     });
 
     test('convert a Map', () {
-      var map = {'a': 1, 'b': 2, 'c': 3};
+      var map = {
+        'a': 1,
+        'b': 2,
+        'c': 3,
+        'd': allowInteropCaptureThis((that) => 42)
+      };
       var jsMap = new JsObject.jsify(map);
       expect(!context.callMethod('isArray', [jsMap]), isTrue);
       for (final key in map.keys) {
         expect(context.callMethod('checkMap', [jsMap, key, map[key]]), isTrue);
       }
     });
 
     test('deep convert a complex object', () {
       dynamic object = {
         'a': [
@@ skipping 293 matching lines @@
           // TODO(jacobr): make this test pass. Currently some type information
           // is lost when typed arrays are passed between JS and Dart.
           // expect(context.callMethod('isPropertyInstanceOf', ['o', listType]),
           //    isTrue);
           context.deleteProperty('o');
         }
       });
     });
   });
 }